Add unique constraint to email branding domains

Does two things:

1. Revert "Revert "Add unique constraint to email branding domain""

This reverts commit af9cb30ef3.

2. Don’t allow empty string in email branding domain

Columns with multiple `null`s can have a uniqueness constraint. Columns
with multiple empty string values are not considered unique.

This commit:
- removes any duplicate empty string values
- casts empty strings to null string any time these columns are updated

---

Squashed into this single commits because these two things are not
atomic as individual commits.

migrations/versions/0223_add_domain_constraint.py

@@ -0,0 +1,28 @@
+"""
+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.execute("""
+        update
+            email_branding
+        set
+            domain = null
+        where
+            domain = ''
+    """)
+    op.create_unique_constraint('uq_email_branding_domain', 'email_branding', ['domain'])
+
+
+def downgrade():
+
+    op.drop_constraint('uq_email_branding_domain', 'email_branding')