There were some concerns with the custom filter migration script dropping a table,
thus making it unsafe to run in a zero-downtime setting. Upstream introduced
a way to run migrations after deployment, so revisit the old migration script to
make use of this.
Completely remove glitch-soc's Keyword Mutes, migrate
existing database records to CustomFilters.
Handling of client-side filters is still not implemented
in the glitch-soc front-end.