Prepare for partial release.

- Default to `Rails::DeprecatedSanitizer` in `ActionView::Helpers::SanitizeHelper`.
- Add upgrade notes.
- Add sanitizer to new applications Gemfiles.
- Remove 'rails-dom-testing' as a dependency.
Inserting a deprecation message here would satisfy those three constraints: https://github.com/rails/rails-deprecated_sanitizer/blob/master/lib/rails/deprecated_sanitizer/html-scanner/html/sanitizer.rb#L7

The subclasses `super` this method (or don't override it) and is therefore called on every `sanitize` call when using the deprecated behavior.

I'll create a PR for this now and cc you on it to review the deprecation message.