method clean_subject_location not implemented correctly

[jrief]

Description

Method clean_subject_location not implemented correctly.

According to the Django docs and to my own experience, a method for cleaning a specific field attribute should not itself call super().clean(). This is because this method is invoked later anyway. In the current implementation the method FileAdminChangeFrom.clean() is called twice, which in my opinion should not happen.

It also sometimes causes problems if users try to replace an image using the Filer's admin image detail view.

Since no functionality changed, I did not add any extra test.

Checklist

• [x] I have opened this pull request against master
• [ ] I have added or modified the tests when changing logic
• [x] I have followed the conventional commits guidelines to add meaningful information into the changelog
• [x] I have read the contribution guidelines and I have joined #workgroup-pr-review on Slack to find a “pr review buddy” who is going to review my pull request.

[jrief]

sorry, unfortunately clean_data['owner'] might not be set. Just found out while testing locally. owner isn't used anyway in validate_upload(…) so using cleaned_data.get("owner") is safer.

[fsbraun]

Where's cleaned_data["owner"] used and how does that affect the PR? Maybe that's another PR?