Adds blank=True to foreignkeys as necessary.
Initial commit of manage resources page.

This is not yet ready; in particular it still lacks creation and
reorganization, and has potentially poor error-handling.  However it
should at least be usable, and I'm putting it here so others can start
working on the other parts of this project, notably the teacher-side
view, which should probably just be a non-editable version of the same.