Collaborative panels

Allow two or more users to edit the same panel that can also be shared.

## Options
- The first user that starts editing the panel blocks the edit for all other users of the same panel (NB: not editable offline + we are keeping a state of the panel that we may not want)
- Show real-time updates (cursor / panel updates)

## Merge management
When saving a panel, send both the hash of the “old version” (the latest version received by the client) as well as the whole “updated version” - When the server receives the old version’s hash, it checks if it matches its current hash, and if so, applies the changes.
If not, then sends back an error to the client saying that the panel cannot be updated.

## Investigation needed
- How merge conflicts should be managed?