Allow diffing changes in wiki history

What does this MR do?

Adds ability to diff changes to wiki pages.

Based on @gwhyte's work on gitlab-foss!5002 (closed). I rebased it, and updated some function calls that were renamed since the original MR was created a few years ago.

Screenshots

Restructured the Wiki "Page history" page, so the "Page version" column links to the revision (instead of showing a Git commit hash), and so the "Commit" column links to a new Changes page.

The new Changes page:

Does this MR meet the acceptance criteria?

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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

Closes #15242 (closed)