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.
