Follow mode: Separate user camera from collaborator camera

[martinRenou]

Currently the follow mode will make the user camera follow the other collaborator camera. We could probably redesign it so that when following someone else's camera we don't change the user camera.

We could have some kind of model/view design where:

• when following someone, we don't change the user camera model, only the view
• that means that if user1 follows user2 and user2 follows user3, user1 will see a static camera because user2's camera is actually not moving