Resolve "Attachments in e-mail notifications are using the wrong URL"

What does this MR do?

Checks context[:only_path] when converting links to user uploads into URLs, creating absolute or relative URLs as appropriate

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

Why was this MR needed?

We thought AbsoluteLinkFilter would do this work for us, but it does not. Without doing this, emails get relative URLs in them which don't work.

Screenshots (if relevant)

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 Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes #41882 (closed)