This PR adds some more nuance to collaborator cursors.
Rather than being timed out or not timed out, a collaborator can now be
`active`, `idle` or `inactive`.
We calculate this based on the difference between the time that has
elapsed since the user's last activity timestamp.
After 3 seconds of inactivity, they go `idle`.
After sixty seconds of inactivity, they are `inactive`.
After any activity, they become `active` again.
When a user is `active`, we always show their cursor.
When a user is `idle`, we hide their cursor if they're following us,
unless they're highlighted
When a user is `inactive`, we hide their cursor unless they're
highlighted.
### Change Type
- [x] `minor`
### Test Plan
1. Find a friend and experiment with inactive times
2. Join a room that includes an inactive cursors; they should be hidden
on load
3. Have people follow you; do their timeouts feel natural?
### Release Notes
- Improve cursor timeouts and hiding logic.
eb6aa9bbe4