feature: Add assets library to issue

What does this MR do?

Adds a list of uploaded assets in a given issue.

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

Line 159 in issue.js where the markup is built for the toggler is lengthy (and would likely benefit from ES6 template strings).

Why was this MR needed?

It's a response to the feature request in !19562, and helps the UX in issues by providing a helpful list of all uploaded images (typically, screenshots). Also: I discussed this with @jschatz1.

What are the relevant issue numbers?

19562

Screenshots (if relevant)

Before:

After:

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).