Add signature verification badge to compare view

What does this MR do?

Attaches a signature verification badge to a commit (when one is signed) on the revision comparison view.

Are there points in the code the reviewer needs to double check?

The specs I added in compare_controller_spec.rb could have been done better, but I do not know the code base well enough to know if I am missing any helpers or such. Any suggestions would be appreciated.

Why was this MR needed?

Issue: #20268 (closed) introduced showing to the user when a commit is signed. It seems like the view fixed in the current change was overlooked during the previous issue.

Screenshots (if relevant)

Before After

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

#44879 (closed)