Merge pull request #17024 from buddhamagnet/amend-welcome-page

Amend welcome page
:nail_care: the CSS
Remove unused code
rails-html-sanitizer is now the default

Also add to upgrading guide a notice about the deprecated sanitizers
No need to install the deprecated_sanitizer gem

It is not being used in our tests
Merge pull request #17058 from eval/issue-17057

Remove internal options from query string of paths
Keep the original implementation to not having to allocate new objects
No need CHANGLOG entry for a bug in a non released version

[ci skip]
Ensure named path params are symbols (Fixes #16958)
Merge pull request #16966 from why-el/symbolize-path-params

Ensure named path params are symbols (Fixes #16958)
[ci skip] Remove reference to config.threadsafe! in guides/configuriing.md
remove extra # in css
Merge pull request #17059 from bronzle/remove_threadsafe_from_guides

[ci skip] Remove reference to config.threadsafe! in guides/configuring.md
Merge pull request #17061 from bronzle/remove_extra_#

remove extra # in css
Revert "Merge pull request #16966 from why-el/symbolize-path-params"

This reverts commit 9d05d6de52871e57bfbf54a60de005e8a5f5b0e4, reversing
changes made to 0863c9248fd47a15e88e05ce4fcd80966684c0e3.

The change in the behaviour reported at #16958 doesn't exist since 4.0
and 4.1 works in the same way
Add +variants to ActionView::FileSystemResolver documentation.
Merge pull request #17070 from felipecvo/master

Add +variants to ActionView::FileSystemResolver documentation.
Don’t require already required modules

abstract_unit.rb requires `action_controller` which [already includes the following lines of code](https://github.com/rails/rails/blob/64fcdce1d3a6a8768ab17f3be144270456814f82/actionpack/lib/action_controller.rb#L2L3):

```ruby
require 'abstract_controller'
require 'action_dispatch'
```
Merge pull request #17071 from claudiob/don-t-require-action-dispatch-twice

Don’t require already required modules
Preparing for 4.2.0.beta2 release