Add borders to images in issues, MRs, and Help.

What does this MR do?

Adds a gray border around images in Issues, MRs, and Help pages.

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

That this doesn't effect other elements or pages that it shouldn't.

Why was this MR needed?

It was hard to differentiate between the actual interface and screenshots of it.

What are the relevant issue numbers?

Fixes #18578 (closed)

Screenshots (if relevant)

Issues/MRs

Before:

After:

Help

Before:

After:

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

cc: @jschatz1