Postgres backend schema management in the appservice

[Gnuxie]

Currently we create tables adhoc

mjolnir/mx-tester.yml

Line 9 in ec4cb2e

 - psql postgres://mjolnir-tester:mjolnir-test@localhost:8083/mjolnir-test-db -c "CREATE TABLE mjolnir (local_part VARCHAR(255), owner VARCHAR(255), management_room TEXT)" 

We could potentially rely on matrix-org/matrix-appservice-bridge#442 for this, but we could handle it ourselves if review is taking too long. This is the same pattern used by the other bridges and can be copied pretty closely.

[Gnuxie]

#364 (comment)

[David Teller]

matrix-org/matrix-appservice-bridge#442 seems to have been merged \o/