Fix joining rooms you have been unbanned from (#15323)

* Fix joining rooms you have been unbanned from

Since forever synapse did not allow you to join a room after you have
been unbanned from it over federation. This was not actually because of
the unban event not federating. Synapse simply used outdated state to
validate the join transition. This skips the validation if we are not in
the room and for that reason won't have the current room state.

Fixes #1563

Signed-off-by: Nicolas Werner <nicolas.werner@hotmail.de>

* Add changelog

Signed-off-by: Nicolas Werner <nicolas.werner@hotmail.de>

* Update changelog.d/15323.bugfix

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Signed-off-by: Nicolas Werner <nicolas.werner@hotmail.de>