Fix /api/exam/status/:filename returning 404 for exams that finished cuts

Fixes a bug with !520 (merged), where exams that have "finished_cuts=True" will show an error in the frontend when opening the side panel. It should not do that.

This bug was caught in staging, I overlooked it when merging !520 (merged).

The cause is get_object_or_404(Exam, filename=filename, finished_cuts=False). This PR removes the filter for finished_cuts=False.

In the future, I would like for this logic to be rewritten, since sending over the whole exam data (when only the claim fields and finished_cuts status is used) to the client feels unnecessary. For example, one improvement is that since exam panel already receives metadata from its parent anyway, we can save requests and make things faster by moving claim status info into metadata and send it ine one go. There's also some UI/UX improvements to be made with the side panel, such as explicitly telling admins that they can't cut it because finished_cuts is already true, etc. These are future issues.