Can't join rooms as a guest over federation until a real user has joined that room from the same homeserver

[matrixbot]

This issue has been migrated from #2065.

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1. Be a guest user
2. Be invited to a room with guest access permitted from a different HS

Join request is 403ed.

This is because synapse does a check for whether guests are allowed before anything else (https://github.com/matrix-org/synapse/blob/474c9aadbe542440bb7cd764a654518b5c36f851/synapse/handlers/room_member.py#L246) and if the invite is from a remote server, it doesn't yet have the guest_access state.