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.

tests/app/db.py

@@ -68,7 +68,7 @@ def create_service(
     research_mode=False,
     active=True,
     email_from=None,
-    prefix_sms=None,
+    prefix_sms=True,
 ):
     service = Service(
         name=service_name,