There was a misunderstanding about how we wanted the feature to work.

There is no need for the extra column.
If a template does not have a default address set - they address will be blank.
Otherwise use the address of the service_letter_contact.

There will be another PR to do a db migration.
Template.service_letter_contact = blank if no service_letter_contacts for service ELSE use set Template.serivce_letter_contact_id = service default service_letter_contact

migrations/versions/0154_revert_0153.py

@@ -0,0 +1,20 @@
+"""
+
+Revision ID: 0154_revert_0153
+Revises: 0153_add_is_letter_contact_blank
+Create Date: 2018-01-05 14:09:21.200102
+
+"""
+from alembic import op
+
+revision = '0154_revert_0153'
+down_revision = '0153_add_is_letter_contact_blank'
+
+
+def upgrade():
+    op.drop_column('templates', 'is_letter_contact_blank')
+    op.drop_column('templates_history', 'is_letter_contact_blank')
+
+
+def downgrade():
+    pass