Add notification status for returned letters

We need to update letter notifications with a new status when DVLA
gives us a list of references for returned letters.

This adds the new status to the models and the DB.

DVLA call this 'returned mail', so I'm using it as the status name
since it seems less ambiguous than 'returned'.

migrations/versions/0224_returned_letter_status.py

@@ -0,0 +1,20 @@
+"""
+
+Revision ID: 0224_returned_letter_status
+Revises: 0223_add_domain_constraint
+Create Date: 2018-08-21 14:44:04.203480
+
+"""
+from alembic import op
+
+
+revision = '0224_returned_letter_status'
+down_revision = '0223_add_domain_constraint'
+
+
+def upgrade():
+    op.execute("INSERT INTO notification_status_types (name) VALUES ('returned-letter')")
+
+
+def downgrade():
+    op.execute("DELETE FROM notification_status_types WHERE name='returned-letter'")