Properly support policy server policies

The following commands would be perfect to properly use policy servers for a lot of administrated rooms:

```
!mjolnir msc4284_set * - Set the policy server for all Mjolnir-protected rooms.
!mjolnir msc4284_set * unset - Unset/disable the policy server for all Mjolnir-protected rooms
```

However, I heard they got removed and a proper implementation for the stable policy server protocol is still missing. It would be very helpful to have these commands in a working form.