Update PREMIS Rights models & forms for mandatory/optional fields

refs #6488

Update blank & null attributes on PREMIS Rights related tables to reflect
which fields were mandatory or optional in the PREMIS spec.  If a field is
optional, but in its own table (eg RightsStatementCopyrightNote), then it
is marked as blank=null=False, since that row will only be created if that
field has information.  Update SQL for those tables to reflect the Django
model.
Quick parse of PREMIS Copyright rights to DB

refs #6488

WIP: Should be updated later