Fix migration sequence (IMPORTANT: see comment)

IMPORTANT: if your database is migrated past djsopnet 0011, you must
either migrate down to 0011 prior to pulling this commit and remigrate
after pulling, or migrate after pulling with the --merge flag. If you
already have user constraints tables in your database from out-of-stream
0006_add_user_constraints, you must either delete those three tables
prior to running the new migration sequence, or manually mark the new
0012_add_user_constraints migration as run.

The user constraint migrations were merged ordered prior to irreversible
migrations, breaking the migration sequence for databases following
blockwise-sopnet. Resequence the creation of user constraints to be
after most major database refactoring.