key-phrase notifications

[novacrazy]

Using https://github.com/postgrespro/rum#rum_tsquery_ops it should be possible to allow users to specify a trigger-phrase for which new messages will create a notification.

It's like the opposite of a full-text search.

However, I need to figure out how to install the extension with Postgres running in Docker, and still be able to update the database eventually using https://github.com/tianon/docker-postgres-upgrade or similar, which may not support having the extension loaded at migration-time, if that's even required.