Feature: add room threads list (#6575)

Add threads list screen for rooms:

- Add `ThreadsListService` to subscribe to thread changes in the room.
- Create `ThreadsListView` and its associated node a presenters (the UI may change).
- Add a menu icon in the room screen to open it.

This is still pending info about unread threads, so several UI components related to it will be hidden.

* Add feature flag and use it to hide the access to this new screen

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Co-authored-by: ElementBot <android@element.io>

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