Fix missing django_admin_log table.

This is required for new installs. admin:0001_initial will rename it.
Old name is required for people:0018_add_fields_person,
people:0019_move_users_to_persons, and people:0020_fix_fields_person
migrations, which run before admin:0001_initial.

Thanks Russell Sim for patch.

Change-Id: I978203386c7db5f319c9041b7f66da19bff0f50f
Skip steps if django_comments does not exist.

Required for migrations to work properly.

Was previously created by the django.contrib.comments module which is no
longer used.

Patch provided by Russell Sim.

Change-Id: Ifeb3b71105c5959702af73e3d3e5749c6d2ab847
Fix pbsmoab migrations not running.

Thanks Russell Sim.

Change-Id: I9dc3d4d9e96a02efbf2a81ddd2b6d35593ced3b7
Fix broken migration depends.

Thanks Russell Sim.

Change-Id: Ic0886e96d94044cbbc7bf74609be6dcb0dc4408e