Update Action Text to use HTML5 when available

[flavorjones]

Motivation / Background

Update Action Text to use the HTML5 sanitizer when available, and to parse markup using Nokogiri::HTML5 when available. Previously, Action Text used Nokogiri's HTML4 parsers for these operations.

Detail

This pull request updates the default value for ActionText::ContentHelper.sanitizer from Rails::Html::Sanitizer.safe_list_sanitizer.new to Rails::Html::Sanitizer.best_supported_vendor.safe_list_sanitizer.new. The method .best_supported_vendor is available in rails-html-sanitizer v1.6.0 and later, which 500ccaa made the minimum for Action View.

This pull request also adds two new methods that return the "best supported" HTML document and document fragment classes: ActionText.html_document_class and ActionText.html_document_fragment_class. These methods are used internally where code previously referenced Nokogiri::HTML directly.

The change from #clone to #dup is necessary to work around an issue in Nokogiri where #clone is not defined properly for HTML5 fragment and the fragment does not have a parent Document. #dup behaves the way we expect, so this should be fine.

Additional information

This is part of ongoing work to update Rails to use HTML5.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[flavorjones]

For posterity: the #clone change in this PR is to work around the upstream issue sparklemotion/nokogiri#2908