WIP: Add an overview of assets to issue pages

CMD-c CMD-v the commit message for this merge

Add an overview of assets to issue pages

This merge request is an initial pass at adding an overview of all assets attached to an issue to the top of the issue page. It parses the images out of the Markdown for the note, and so relies on said Markdown being correctly formatted. This will fail silently on error, which is less wrong that failing completely, but still should be handled better.

This commit also fails to refresh the UI when adding a new asset, as adding this ability is better encompased in amending the notes submission method to POST and GET a JSON payload, and render it to HTML client side, rather than using the current method of providing a note_html endpoint, and dumping the rendered HTML into the DOM.

• 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).