View & edit my reason after sending knock

### Your use case

#### What would you like to do?
I would like to see the reason i submitted when I asked to join a room with join rule = knock
#### Why would you like to do it?
I want to make sure I submitted the correct reason when one is required, e.g. by community standards. I want to edit it, if not.
#### How would you like to achieve it?
The knock-sent-UI should show the reason and have a button that enables editing. The button is there so it is clear that the reason was already sent. Upon editing, the UI is the same or very similar to the one without prior knocks sent.

On the room admin side, the reason updates silently without further notifications.

### Have you considered any alternatives?

Updating the reason may also mark knocks as unseen again, but I don't think that exists for current knocks either.

### Additional context

https://github.com/vector-im/element-web/issues/18655

Start with the linked Element issue #18655, then trace the knock-sent UI and the room-admin handling described here. Done means the submitted reason is visible after sending, can be edited with a clear state change, and updates silently on the admin side without additional notifications.

Written by the indexing model from the issue text.