WIP: Resolve "Add assets library to issue"

What does this MR do?

Adds a list of assets in the IssuesController#show action, to have an overview of all the attached files in an issue.

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

I'm not utterly happy with the method to extract the assets from the issues. I haven't found any model with the attachments of an issue (it seems we just upload the file and then link it).

I want to ask in this review for a better way of doing this. Once this is done we can add the proper unit tests in the backend.

What are the relevant issue numbers?

19562

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together Closes #19562 (moved)