Ability to "undo" a protected file

**Is your feature request related to a problem? Please describe.**
Currently, if you select a course/membership to protect a file, there's no way to undo that.

**Describe the solution you'd like**
If the course/membership value is cleared for protected file, "unprotect" the file. We'd need to have a note about where the file was originally located.

**Describe alternatives you've considered**
We may want to not support this since:

a) the file might have been re-uploaded already and still exist
b) Links within existing documents wouldn't be automatically updated, or we'd need to do some kind of search/replace