Include the topic event in the prejoin state, per MSC3173. (#11666)

Invites and knocks will now include the topic in the stripped state
send to clients before joining the room.

docs/sample_config.yaml

@@ -1488,6 +1488,7 @@ room_prejoin_state:
    # - m.room.encryption
    # - m.room.name
    # - m.room.create
+   # - m.room.topic
    #
    # Uncomment the following to disable these defaults (so that only the event
    # types listed in 'additional_event_types' are shared). Defaults to 'false'.