AIP Reingest: DC metadata marked when updated

refs #6488

DC metadata should be written to the METS file only if it has been
changed during reingest. To track this, add a status field to the
DublinCore table which can be ORIGINAL, REINGEST or UPDATED.  ORIGINAL
is the default for AIPs being created normally, REINGEST is the status
when it is initially parsed to the DB, UPDATED is when the data is
changed after reingest.

Fix DublinCore.metadataappliestotype to be a foreign key.
Clean up PREMIS Rights tables

Clean up imports so only importing django.forms, instead of that and
several more specific things. Fix RightsStatement to use choices, and
update form for that. Update title text to be grammatical.