Set branding_type to org if it’s none

So later we can:
- make it non-nullable later
- remove `govuk` as an option

This is mostly for people’s local databases, the manual work here has
been done on production already.

migrations/versions/0217_default_email_branding.py

@@ -0,0 +1,25 @@
+"""
+ Revision ID: 0217_default_email_branding
+Revises: 0216_remove_colours
+Create Date: 2018-08-24 13:36:49.346156
+ """
+from alembic import op
+from app.models import BRANDING_ORG
+
+revision = '0217_default_email_branding'
+down_revision = '0216_remove_colours'
+
+
+def upgrade():
+    op.execute("""
+        update
+            email_branding
+        set
+            brand_type = '{}'
+        where
+            brand_type = null
+    """.format(BRANDING_ORG))
+
+
+def downgrade():
+    pass