Add a column for a service to set SMS prefixing

In future changes, services will be able to control whether their text
messages will be prefixed with the name of their service.

This commit:
- adds a column to store the value of that setting
- makes the service model take notice of it, if it were to have a value
  set

It doesn’t:
- provide a way of setting the value of this column

Currently the column can have three values:
- `None` – ignore it (this is what all current services will start as)
  and continue to determine whether to prefix messages by looking at the
  sender
- `True` – always the service name to the start of text messages
- `False` – never add the service name to the start of text messages

In the future we’ll migrate all services to be either `True` or `False`,
the `None` will go away and all services will have direct control over
the setting.

migrations/versions/0132_add_sms_prefix_setting.py

@@ -0,0 +1,23 @@
+"""
+
+Revision ID: 0132_add_sms_prefix_setting
+Revises: 0131_user_auth_types
+Create Date: 2017-11-03 11:07:40.537006
+
+"""
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+revision = '0132_add_sms_prefix_setting'
+down_revision = '0131_user_auth_types'
+
+
+def upgrade():
+    op.add_column('services', sa.Column('prefix_sms', sa.Boolean(), nullable=True))
+    op.add_column('services_history', sa.Column('prefix_sms', sa.Boolean(), nullable=True))
+
+
+def downgrade():
+    op.drop_column('services_history', 'prefix_sms')
+    op.drop_column('services', 'prefix_sms')