Auto-join rooms when knock accepted

This issue has been migrated from [#16307](https://github.com/matrix-org/synapse/issues/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...

https://github.com/matrix-org/synapse/assets/89144281/add41427-9542-4e14-aab0-7e81eb931445

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...

Start by tracing Synapse's server-side handling of accepted knocks and the transition to room membership; the issue names no files, tests, or entry points. Confirm the current flow and define completion as automatically joining a user after their knock is accepted, without requiring a second join action.

Written by the indexing model from the issue text.