We need to suppress systemd service-stopping requests in certain rare
cases like https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/771

That issue seems to describe a case, where a migration from mxisd to
ma1sd was happening (DB files had just been moved), and then we were
attemping to stop `matrix-ma1sd.service` so we could import that database into
Postgres. However, there's neither `matrix-mxisd.service`, nor
`matrix-ma1sd.service` after `migrate_mxisd.yml` had just run, so
stopping `matrix-ma1sd.service` was failing.

In cases where pgloader is not enough and we need to do some additional
migration work after it, we can now use
`additional_psql_statements_list` and
`additional_psql_statements_db_name`.

This is to be used when migrating `matrix-registration`'s data at the
very least.