Enable realtime editing for BlockNote editor

The first version has some limitations:

• you need to force edit lock
• the effective author for the collaboration session is not computed
• no auto-save
• it's still using the old edit form toolbar, not the new realtime toolbar (that CKEditor is using)
• partial clustering support: the state is synchronized between cluster nodes when all nodes are up, but there is no synchronization performed when a node is restarted or reconnected (we need to add custom events for this and ask the cluster leader to give the collaboration state)
• the Yjs WebSocket doesn't keep an authoritative Yjs document because it's not easy to use Yjs API server-side in Java; in order to perform the initial sync from the Yjs protocol, when a new client connects, it records all update messages and replays them to connecting users; this works, but it's far from optimal: instead of sending only the diff (missing changes) we basically send all changes; besides bandwidth and connection time, this could also take server-side memory for collaboration sessions with a long life.