BlockNote realtime sessions save automatically

Content edited with the BlockNote editor in a realtime collaboration session is now saved automatically, about once a minute while there are unsaved changes. This closes the "there is no auto-save" limitation announced when BlockNote realtime collaboration was introduced.

Only one of the collaborators' browsers performs each save, so the session adds a single version per save instead of one version per participant. That version records the summary "Auto-saved during real-time collaboration", the same summary a CKEditor realtime session uses. Saving by hand keeps working and takes precedence over the automatic save.

The save status ("Unsaved", "Saving", "Saved") that the realtime toolbar displays is not shown in the BlockNote action bar yet, so there is currently no on-screen indication of when the automatic save happens. We plan to add it in the next version.