Add unique constraint to email branding domain

Brandings with a domain set should be considered canonical. It doesn’t
make sense to have the same domain set on multiple different email
brands – you can’t tell which one to use.

migrations/versions/0223_add_domain_constraint.py

@@ -0,0 +1,20 @@
+"""
+Revision ID: 0223_add_domain_constraint
+Revises: 0222_drop_service_branding
+Create Date: 2018-08-24 13:36:49.346156
+ """
+from alembic import op
+
+
+revision = '0223_add_domain_constraint'
+down_revision = '0222_drop_service_branding'
+
+
+def upgrade():
+
+    op.create_unique_constraint('uq_email_branding_domain', 'email_branding', ['domain'])
+
+
+def downgrade():
+
+    op.drop_constraint('uq_email_branding_domain', 'email_branding')