`travel/travel_to` travel time helpers, now raise on nested calls

Merge pull request #25704 from fledman/add_respond_to_missing_to_rails_application_configuration_custom

implement respond_to_missing? to match method_missing

Use `squish` rather than `strip_heredoc`

Allow rake notes to work with other directories.

Deprecate usage of nil as route path

In Rails 4 these kind of routes used to work:

```ruby
scope '/*id', controller: :builds, as: :build do
  get action: :show
end
```

But since 1a830cbd, routes are only created for
paths specified as strings or symbols. Implicit `nil` paths are just ignored,
with no deprecation warnings or errors. Routes are simply not created. This come
as a surprise for people migrating to Rails 5, since the lack of logs or errors
makes hard to understand where the problem is.

This commit introduces a deprecation warning in case of path as `nil`, while
still allowing the route definition.

Display railtie class name in `rails initializers`

accidently set it to.

Made the change and test still passes.

Add header demarcation to Action Cable guide

Guide generation is configured to detect a guide header by a 40-or-more-hyphen long line. The Action Cable guide was missing this line, so the page title on the guides site was only showing "Ruby on Rails Guides", and the section title in the Kindle index was "Ruby on Rails Guides". Adding the header demarcation line back in fixes both of these.

Fixes #25697

[ci skip]

Point to the correct documentation

The options used in this file are *not* directly described in the Rails 5.0
release notes (http://edgeguides.rubyonrails.org/5_0_release_notes.html), but
instead in section 2 of the Guide for Upgrading Ruby on Rails document
(http://edgeguides.rubyonrails.org/upgrading_ruby_on_rails.html).

Robots.txt removed disallow all instructions

Additional directories can be added using
SourceAnnotationExtractor::Annotation.register_directories("spec", "other_dir")

Result: rake notes will now extract notes from these directories.

Dry downloader

Pathname might not be always initialized.

Fixed unnecessary nesting in handle_dependency

Fix a link to use RDoc syntax and make sure that the titles' level
match the section we are in since we are both documenting the new
and the legacy syntax.

fix respond_to? documentation [ci skip]

This nested if checked the same value as the containing case statement.
Moved the code in the if/else into when/else in the containing case.

Require 'pathname' explicitly

Update Syntax Highlighter to version 4 [ci skip]

Add ActiveSupport::Notifications hook to ActionCable::Server.broadcast

ActionCable, sometimes add_channel is not called.

Improve error message when record touch fails.

- Build Syntax Highlighter based on https://github.com/syntaxhighlighter/syntaxhighlighter/wiki/Building and use theme generated by fxn.
- This also bundles these brushes in the file itself- ruby,css,xml,sql,javascript,plain. Generation command used:
gulp build --compat --brushes=javascript,ruby,xml,sql,plain --theme=/Path/To/RailsSource/rails/guides/assets/stylesheets/syntaxhighlighter/shThemeRailsGuides.css
- rm all old js and css files
- Moved over how JS configuration is created for Syntax Highlighter
- Fixed layout for all imports
- Fixes bug with old copy paste, generating wrong space character on webkit https://github.com/syntaxhighlighter/syntaxhighlighter/blob/7caa9632e7fe470d944fc06b4db7d35352ebaa36/src/dom.js#L227

 Fixes #25655

Clarify Postgres initials. [skip ci]

Update Rails 5 release notes with syntax fixes. [ci skip]

The current error message only indicates that a touch can fail because the record is new. In practice, we saw cases where touches were failing because the record had been destroyed. `persisted?` checks `new_record?` *and* `destroyed?`. It was confusing to get a message about a new record when in reality we were destroying records.

I also included a helpful tip for users to consider using `persisted?`, `new_record?`, or `destroyed?` before touching.

     as this can lead to confusing time stubbing.

     Instead of:

         travel_to 2.days.from_now do
           # 2 days from today
           travel_to 3.days.from_now do
             # 5 days from today
           end
         end

     preferred way to achieve above is:

         travel_to 2.days.from_now
         # 2 days from today

         travel_back
         travel_to 5.days.from_now
         # 5 days from today

Closes #24690
Fixes #24689

Fix typo: accidently -> accidentally. [ci skip]