Room membership: joinRoomById

[chrisabruce]

Start the requesting user participating in a particular room.

Note that this API requires a room ID, not alias. /join/{roomIdOrAlias} exists if you have a room alias.

This API starts a user participating in a particular room, if that user
is allowed to participate in that room. After this call, the client is
allowed to see all current state events in the room, and all subsequent
events associated with the room until the user leaves the room.

After a user has joined a room, the room will appear as an entry in the
response of the |/initialSync|_ and |/sync|_ APIs.

If a third_party_signed was supplied, the homeserver must verify
that it matches a pending m.room.third_party_invite event in the
room, and perform key validity checking if required by the event.