Use a popover for issue references
This changes issue references to use a popover similar to MR references. This results in data that is always up-to-date and we plan on adding extra information to the popover in future iterations.
The issue popover frontend has already been deployed in !88128 (merged). So the frontend can handle both old and new Markdown HTML.
Related to #25607 (closed)
Screenshots or screen recordings
Edited by Heinrich Lee Yu