End to bridge encryption implementation

So far this is passing my basic tests, but could use some testing from people
that are much more familiar with how this is supposed to work.

Refs #27

database/migrations/08-add-crypto-store-to-database.sql

@@ -0,0 +1,3 @@
+-- This migration is implemented in migrations.go as it comes from
+-- maunium.net/go/mautrix/crypto/sql_store_upgrade. It runs upgrade at index 0
+-- which is described as "Add crypto store to database".