Unify the way we decide whether a room is a DM or a group room (#3100)
* Add centralised 'room is DM' check
Also add extension functions for `MatrixRoom` and `MatrixRoomInfo`.
* Use the centralised method and extension functions through the app, including:
- Room list.
- Room details screen.
- Invites.
- Notifications.
Replace most `isDirect` usages with `isDm`.
* Update screenshots
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Co-authored-by: ElementBot <benoitm+elementbot@element.io>