Better cleanup functionality

[kerchner]

SFM seems to lack facilities to more easily get rid of content that is no longer needed.

Users cannot:

• Delete an export
• Delete all of their exports
• Delete a collection set
  • this should also delete the associated harvests, WARCs, historical objects, collections, seeds, etc.
• Delete a collection (but they can deactivate it)
  • this should also delete the associated harvests, WARCs, historical objects, seeds, etc.

Administrators:

• Can delete objects via the Django admin UI. Django admin UI provides sorting by date, by user, etc., so this should reasonably suffice for admins.

Related, existing issues:

• Deleting seeds leaves orphaned historical seeds #847 - I assume this bug still exists, but someone may want to re-test it
• Change labels for "deactivate" functionality #1062

If we opt not to implement a certain "cleanup" feature in the UI, we should verify that the back-end procedure is adequately written up in the documentation (i.e. the readthedocs site)

We can create individual tickets for the above after discussing all of this as a whole.