Why:

* When getting an error that generates a screenshot it would be helpful
  to be able to ctrl+click it to quickly open it in the browser, which
  does not work with relative paths

This change addresses the need by:

* Changing `image_path` to disregard the relative path and use the
  absolute one instead

Found several instances.

Follow up on 63d530c5

Because the UJS library creates a script tag to process responses it
normally requires the script-src attribute of the content security
policy to include 'unsafe-inline'.

To work around this we generate a per-request nonce value that is
embedded in a meta tag in a similar fashion to how CSRF protection
embeds its token in a meta tag. The UJS library can then read the
nonce value and set it on the dynamically generated script tag to
enable it to execute without needing 'unsafe-inline' enabled.

Nonce generation isn't 100% safe - if your script tag is including
user generated content in someway then it may be possible to exploit
an XSS vulnerability which can take advantage of the nonce. It is
however an improvement on a blanket permission for inline scripts.

It is also possible to use the nonce within your own script tags by
using `nonce: true` to set the nonce value on the tag, e.g

    <%= javascript_tag nonce: true do %>
      alert('Hello, World!');
    <% end %>

Fixes #31689.

Before:
```
~/.rbenv/versions/2.5.0/bin/ruby -w -Itest -Ilib -I../activesupport/lib -I../actionpack/lib -I../actionview/lib -I../activemodel/lib test/application/routing_test.rb
Run options: --seed 5851

.......~/code/rails/actionpack/lib/action_dispatch/routing/route_set.rb:156: warning: method redefined; discarding old custom_path
~/code/rails/actionpack/lib/action_dispatch/routing/route_set.rb:321: warning: previous definition of custom_path was here
~/code/rails/actionpack/lib/action_dispatch/routing/route_set.rb:162: warning: method redefined; discarding old custom_url
~/code/rails/actionpack/lib/action_dispatch/routing/route_set.rb:321: warning: previous definition of custom_url was here
....~/code/rails/actionpack/lib/action_dispatch/routing/route_set.rb:156: warning: method redefined; discarding old custom_path
~/code/rails/actionpack/lib/action_dispatch/routing/route_set.rb:321: warning: previous definition of custom_path was here
~/code/rails/actionpack/lib/action_dispatch/routing/route_set.rb:162: warning: method redefined; discarding old custom_url
~/code/rails/actionpack/lib/action_dispatch/routing/route_set.rb:321: warning: previous definition of custom_url was here
~/code/rails/actionpack/lib/action_dispatch/routing/route_set.rb:156: warning: method redefined; discarding old custom_path
~/code/rails/actionpack/lib/action_dispatch/routing/route_set.rb:321: warning: previous definition of custom_path was here
~/code/rails/actionpack/lib/action_dispatch/routing/route_set.rb:162: warning: method redefined; discarding old custom_url
~/code/rails/actionpack/lib/action_dispatch/routing/route_set.rb:321: warning: previous definition of custom_url was here
..........~/code/rails/actionpack/lib/action_dispatch/routing/route_set.rb:156: warning: method redefined; discarding old custom_path
~/code/rails/actionpack/lib/action_dispatch/routing/route_set.rb:321: warning: previous definition of custom_path was here
~/code/rails/actionpack/lib/action_dispatch/routing/route_set.rb:162: warning: method redefined; discarding old custom_url
~/code/rails/actionpack/lib/action_dispatch/routing/route_set.rb:321: warning: previous definition of custom_url was here
~/code/rails/actionpack/lib/action_dispatch/routing/route_set.rb:156: warning: method redefined; discarding old custom_path
~/code/rails/actionpack/lib/action_dispatch/routing/route_set.rb:321: warning: previous definition of custom_path was here
~/code/rails/actionpack/lib/action_dispatch/routing/route_set.rb:162: warning: method redefined; discarding old custom_url
~/code/rails/actionpack/lib/action_dispatch/routing/route_set.rb:321: warning: previous definition of custom_url was here
.....

Finished in 13.233638s, 1.9647 runs/s, 5.8185 assertions/s.
26 runs, 77 assertions, 0 failures, 0 errors, 0 skips
```

After:
```
~/.rbenv/versions/2.5.0/bin/ruby -w -Itest -Ilib -I../activesupport/lib -I../actionpack/lib -I../actionview/lib -I../activemodel/lib test/application/routing_test.rb
Run options: --seed 38072

..........................

Finished in 12.009632s, 2.1649 runs/s, 6.4115 assertions/s.
26 runs, 77 assertions, 0 failures, 0 errors, 0 skips
```

Although the spec[1] is defined in such a way that a trailing semi-colon
is valid it also doesn't allow a semi-colon by itself to indicate an
empty policy. Therefore it's easier (and valid) just to omit it rather
than to detect whether the policy is empty or not.

[1]: https://www.w3.org/TR/CSP2/#policy-syntax

This reverts commit 86f7c269, reversing
changes made to 5ece2e4a.

If a policy is set then we should generate it even if it's empty.
However what is happening is that we're accidentally generating an
empty policy when the initializer is commented out by default.

`Rails.application.config.content_security_policy` is configured with no
policies by default. In this case, Content-Security-Policy header should
not be generated instead of generating the header with no directives.
Firefox also warns "Content Security Policy: Couldn't process unknown
directive ''".

Skipping over 2.4.0 to sidestep the `"symbol_from_string".to_sym.dup` bug.

References #32028

* Global ignores at toplevel .gitignore
* Component-specific ignores in each toplevel directory
* Remove `actionview/test/tmp/.keep` for JRuby

```
rm actionview/test/tmp/ -fr
cd actionview/
bundle exec jruby -Itest test/template/digestor_test.rb
```

Related to #11743, #30392.

Closes #29978.

Some attr_readers should be `protected` instead of `private`

See https://travis-ci.org/rails/rails/builds/342800276

Some places we can't remove because Ruby still don't have a method
equivalent to strip_heredoc to be called in an already existent string.

Rails 6 will only support Ruby >= 2.3.

Before:

Total allocated: 209050523 bytes (2219202 objects)
Total retained:  36580305 bytes (323462 objects)

After:

Total allocated: 209180253 bytes (2222455 objects)
Total retained:  36515599 bytes (321850 objects)

---

Modest saving of 1612 RVALUEs in the heap on Discourse boot

The larger the route file the better the results.

Saving will only be visible on Ruby 2.5 and up.

This reverts commit f282f375.

This reverts commit 9f65d2a0, reversing
changes made to 96684373.

This broken a lot of tests.

Per: https://bugs.ruby-lang.org/issues/13077 String @- will dedupe strings. 

This takes advantage of this by deduping route fragments that are full of duplication usually. 

For Discourse:

Before:

Total allocated: 207574305 bytes (2214916 objects)
Total retained:  36470010 bytes (322194 objects)

After 

Total allocated: 207556847 bytes (2214711 objects)
Total retained:  36327973 bytes (318627 objects) <- object that GC can not collect


So we save 3500 or so RVALUES this way, not the largest saving in the world, but worth it especially for large route files.

Before, if the application defined after an engine this method would not
recognize the route since it was not defined insdie the engine.

:tada::tada::tada:

It makes sense to be as strict as possible
with headers from the outside world,
but allowing @ to support Apache's mod_unique_id
(see #31644) seems OK to me

Fixes #31823.

[ci skip]

### Summary

This PR changes .rubocop.yml.

Regarding the code using `if ... else ... end`, I think the coding style
that Rails expects is as follows.

```ruby
var = if cond
  a
else
  b
end
```

However, the current .rubocop.yml setting does not offense for the
following code.

```ruby
var = if cond
        a
      else
        b
      end
```

I think that the above code expects offense to be warned.
Moreover, the layout by autocorrect is unnatural.

```ruby
var = if cond
  a
      else
        b
      end
```

This PR adds a setting to .rubocop.yml to make an offense warning and
autocorrect as expected by the coding style.
And this change also fixes `case ... when ... end` together.

Also this PR itself is an example that arranges the layout using
`rubocop -a`.

### Other Information

Autocorrect of `Lint/EndAlignment` cop is `false` by default.
https://github.com/bbatsov/rubocop/blob/v0.51.0/config/default.yml#L1443

This PR changes this value to `true`.

Also this PR has changed it together as it is necessary to enable
`Layout/ElseAlignment` cop to make this behavior.

- Update the default HSTS max-age value to 31536000 seconds (1 year)
  to meet the minimum max-age requirement for https://hstspreload.org/.

[ci skip]