Decorate rich_text with word-diff highlights

What does this MR do?

Run diff_lines through Gitlab::Diff::Highlighter#highlight in order to apply word-diff highlights to the contents of the rich_text field, revealing word/character level differences between lines. Display of this relies on a change to FE code to switch from display the context of the text field to the rich_text field.

Screenshots

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Saved DiffNote with Suggestion

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Addresses #26543 (closed)