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.
Maybe you could write:

> Two new scrubbers have also been added: `PermitScrubber` and `TargetScrubber`. For further information, you can read the [gem's readme](https://github.com/rails/rails-html-sanitizer).

Also could you please wrap your additions around 80 chars ? Thank you so far ! :-)