Rework collaborative session UI/flow

Rework collaborative session UI/flow: Proposed flow:
• _Start live collaboration_
- Save the room link so you can access it whenever you want.
- The room link persists after the end of your collaborative session.
- Restarting the collaborative session will generate a new link to a new room.
- Your collaborative sessions are private thanks to end-to-end encryption.
- _Start session_
- Share this link with anyone you want to collaborate with: [link] [your-name]
- _Stop session_