Allow comment creators to fix/drop

So far, only the owner of the review request can fix/drop issues.

This commit allows the creator of a comment to fix/close it themselves.
Fixes #2888.

API: Allow comment *creators* to fix/drop

UI: Show Fixed/Drop buttons for issue creators

Tried with local instance:
- review owner can still fix/drop issues
- issue creators can successfully fix/drop issues
- other people can not fix/drop issues as before

All tests pass.


I updated the following tests to work again and also test if the above three properties hold:
- test_update_diff_comment_issue_status
- test_update_screenshot_comment_issue_status