Changing labels/milestones during editing issue resets text

[MGChecker]

I understand why this happens, but if one does this accidentally after writing a lot of text, it can be quite frustrating.

• Gitea version (or commit ref): 1.14.5
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (provide example URL)
  • No

[silverwind]

Comment text should be preserved in localStorage during page reloads. It could probably be keyed off the URL.

Edit: actually some pages like review can have multiple comment boxes on one page, so a more elborate keying is needed.

[jolheiser]

Duplicate of #8342