Avoid page reloads on unchanged assignee/labels

[silverwind]

STR:

1. View any issue
2. Double click on "Assignees" or "Labels" in the sidebar, e.g. don't change anything
3. Page unnecessarily reloads

Should ideally also eliminate the page reloads when values actually change and just update DOM accordingly in JS.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. I am here to help clear issues left open even if solved or waiting for more insight. This issue will be closed if no further activity occurs during the next 2 weeks. If the issue is still valid just add a comment to keep it alive. Thank you for your contributions.

[silverwind]

👍

[jolheiser]

This is mostly fixed by #23014
It doesn't cover the "update the DOM" case, please re-open if you think it should be part of this issue or if we should have a specific issue for it now that this is partially fixed.

[wxiaoguang]

I also agree to "just update DOM accordingly in JS", but those dropdowns are much more complex than I thought, so I left them as they were. To touch them .... it needs another (big) refactoring PR.

[silverwind]

Should probably rewrite them in Vue/React/... 😉