Introduce `Room.hasEncryptionStateEvent` (#4030)
... and replace a lot of calls to `MatrixClient.isRoomEncrypted` with it.
This is a lesser check (since it can be tricked by servers withholding the
state event), but for most cases it is sufficient. At the end of the day, if
the server witholds the state, the room is pretty much bricked anyway. The one
thing we *mustn't* do is allow users to send *unencrypted* events to the room.
