Sync: join the realtime channel before editing the map

[almet]

Currently, when joining an editing session, the user sees the map as it was saved, but all the operations that happened after the save by other peers aren't reflected in the interface.

Instead, we could join the session if we have the sufficient permissions, and even if we're not in edit mode.

[almet]

Right now, this is not implemented to avoid triggering too many websocket connections, in the case a map is shared with wide audience with "everybody can edit" permission.