Persistent change tracking

One limitation of https://github.com/nextcloud/text/pull/1296 is that we cannot persist changes when an editing session is ended (all users close the file) or if the file gets changed externally.

• See if we can merge outside changes into an existing editing session
  • We would need to generate a diff of the two documents and convert that into a proper prosemirror transaction that we can apply on top of our existing steps
  • This would also resolve having a dedicated conflict resolution dialog
• If we decide to keep steps forever (for persistent annotation purposes for example) we need to find a way to reduce the amount of steps, either by compressing the steps into only those handled by different authors or by somehow storing the author information in the document tree.
• Ideally all that should happen in the backend rather than the frontend, however this would require having a PHP rewrite of prosemirror-transform and schema handling for being able to work with documents and the transformation steps.

Review pull request 1296 first, then trace how editing sessions, external file changes, and transformation steps are currently handled. Evaluate the proposed ProseMirror transaction path against a backend PHP approach using prosemirror-transform and schema handling. Done means the persistence and outside-change behavior has a defined, implementable design.