Correct Maintenance Policy for Rails 5.2

[ci skip]

Update bug report templates

Set Rails 5.2.0 in templates

Only disable headless chrome gpu on Windows

This was causing single db applications to have rake tasks named
`db:create:primary`. These tasks are only useful to multiple database
applications so they shouldn't be generated.

I realized I wasn't really testing some of the new rake tasks added so I
built out this new test that uses a multi-db database.yml and allows us
to run create/drop/migrate/schema:dump/schema:load and those that are
namespaced like create:animals. This will make our testing more robust
so we can catch problems quicker and set a good place to add future
tests as these features evolve.

Pass the spec name to load_schema in order to load from the correct
structure file when there are multiple databases

Improve the null origin error message

Avoid generating full changes hash on every save

`changed_attribute_names_to_save` is called in `keys_for_partial_write`,
which is called on every save when partial writes are enabled.

We can avoid generating the full changes hash by asking the mutation
tracker for just the names of the changed attributes. At minimum this
saves one array allocation per attribute, but will also avoid calling
`Attribute#original_value` which is expensive for serialized attributes.

Fix leftover references to VariantsController in AS documentation [ci skip] 

VariantsController has been merged to RepresentationsController, this
PR fixes outdated references to VariantsController in ActiveStorage documentation.

Mocha updates

Because this class includes not only `assert_difference` but also tests
of other assertion methods.

Enable verbose option in ActiveStorage

rewords a bit RDOC_MAIN.rdoc [ci skip]

* Same change as bba832d4

Per Chromium team this has not been necessary on other platforms for quite some time: https://bugs.chromium.org/p/chromium/issues/detail?id=737678#c1

[LinkedRach, Daniel Colson, Xavier Noria]

Update URLs in RELEASING_RAILS.md [ci skip]

* Use https instead of http with URI scheme for Travis
* Update Url for "Agile Web Development with Rails"

Mocha v1.5.0 is currently the latest release.

This has been possible since Mocha v1.0 and makes it clear that we want
Mocha to integrate with Minitest, not Test::Unit.

The `require: false` option hasn't been needed since Mocha v1.0.

Fix `actionview/CHANGELOG.md`

[ci skip]

- Add missing dots at the end of sentences.
- Wrap RecordTagHelper into `.
- `RecordTagHelper` => `ActionView::Helpers::RecordTagHelper`.

[ci skip]

Remove RecordTagHelper

Use an ActiveSupport::CurrentAttributes model to provide the host for service urls

* Since #18411, we started to inform about extracted gem (record_tag_helper)
  to developers who use `ActionView::Helpers::RecordTagHelper` 's methods.

* Currently, it seems no problem that we don't have to support no longer.

The `Capybara.server=` proc acceptance restored in Capyara 3.0.1.
Ref: https://github.com/teamcapybara/capybara/commit/8f115d94e035eca992036f16e50c1dce5f555c97

Trying to pass the current request down to the service so that it can
create full urls instead of paths makes the API messy so use a model
based on ActiveSupport::CurrentAttributes to provide the current host
to services that need it (primarily the disk service).

Before it was possible to for example use the direct upload controller
without using the site.

This way we don't need to remember which packages we need to install.

[ci skip] Add securing rails app guide link to credential section

* In 5.2 release note, added [securing rails app guide](http://edgeguides.rubyonrails.org/security.html#custom-credentials) link to [credentials section](http://edgeguides.rubyonrails.org/5_2_release_notes.html#credentials).

Bring back private class methods accessibility in named scope