join_authorised_via_users_server user must be joined to the room

[turt2live]

Link to problem area: https://spec.matrix.org/v1.6/rooms/v10/#authorization-rules (and room versions using the same condition)

Issue
Under 4.3.5.2 we only verify that the user has permission to do it, but in practice we also pull their membership event to ensure they're also joined to the room.

This is a documentation error: implementation and the original MSC both require the referenced user.

Implementation (Synapse): https://github.com/matrix-org/synapse/blob/a368d30c1cfe7457fca4fcdd03ae481ba65a226c/synapse/event_auth.py#L632

The MSC states:

It should be confirmed that the authorising user is in the room. (This prevents situations where any homeserver could process the join, even if they weren't in the room, under certain power level conditions.)

Additionally, the auth_events restriction is not mentioned in the spec:

The auth chain of the join event needs to include events which prove the homeserver can be issuing the join. This can be done by including:

• The m.room.power_levels event.
• The join event of the user specified in join_authorised_via_users_server.

but is added by Synapse here