Make SMS prefix setting non-nullable

We have now:
- defaulted new services to start with this column set to `true`
- migrated all preexisting services[1] to have either `true` or `false` set
  for this column

There is no way for a service to switch back from `true`/`false` to
`null`.

This means that we can safely enforce a non-nullable constraint on this
column now.

1. There is a little gotcha: the GOV.UK Notify service still relies on
   the `sms_sender` column. It doesn’t have a row in the
   `service_sms_senders` table. This means the previous migration
   never changed this service’s value for `prefix_sms` from `null`. So
   this commit also changes its value to `False`, so that the rest of
   the migration, of the whole column, is possible.

migrations/versions/0140_sms_prefix_non_nullable.py

@@ -0,0 +1,46 @@
+"""
+
+Revision ID: 0140_sms_prefix_non_nullable
+Revises: 0139_migrate_sms_allowance_data
+Create Date: 2017-11-07 13:04:04.077142
+
+"""
+from alembic import op
+from flask import current_app
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+revision = '0140_sms_prefix_non_nullable'
+down_revision = '0139_migrate_sms_allowance_data'
+
+
+def upgrade():
+
+    op.execute("""
+        update services
+        set prefix_sms = false
+        where id = '{}'
+    """.format(current_app.config['NOTIFY_SERVICE_ID']))
+
+    op.alter_column(
+        'services',
+        'prefix_sms',
+        existing_type=sa.BOOLEAN(),
+        nullable=False,
+    )
+
+
+def downgrade():
+
+    op.alter_column(
+        'services',
+        'prefix_sms',
+        existing_type=sa.BOOLEAN(),
+        nullable=True,
+    )
+
+    op.execute("""
+        update services
+        set prefix_sms = null
+        where id = '{}'
+    """.format(current_app.config['NOTIFY_SERVICE_ID']))