Auto-join rooms when knock accepted

[matrixbot]

This issue has been migrated from #16307.

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Knocking/Ask to join is being implemented in Element Web. We've hit a flow that we think needs server support to fix.

Knocking is: A type of room allows users to ask-to-join (instead of waiting for an invite), once a user has knocked the admins in the room receive a notification, the new user is accepted to the room and they should automatically be joined.

Issue: Today, once accepted to the room the user is asked to join the room...

266297421-c60a12b3-2f34-48df-abfb-72d82ff1fde6.mp4

You can see this at 24s in the video.

Ideally - if the user has knocked and been accepted they are then joined to the room - they should not have to click 'join' again...

[MadLittleMods]

Relevant comment from original issue:

As per the spec:

Homeservers are permitted to automatically accept invites as a result of knocks as they should be aware of the user’s intent to join the room.

https://spec.matrix.org/v1.8/client-server-api/#knocking-on-rooms

-- @t3chguy, matrix-org/synapse#16307 (comment)