Y.js backend drop in replacement

[juliushaertl]

Use the yjs based collaboration extension with the existing server API (+small modifications).
Turn the sync service provider experiment into a working solution.

• send and store steps encoded as base64
• remove the DB lock when pushing changes to the server
• write tests for joining an existing editing session and collaborating
• write tests for opening an existing md file and collaborating on it
• test performance under load - maybe during contributor week
  • Lets do another one after the merge

[max-nextcloud]

Next steps afterwards...

• put compressed update on server when saving file
• understand awareness protocol
• make use of awareness
• Editing cursor #90
• Bring back author colors #3731
• Using notify_push for syncing #1805
• Only store last awareness messages on the backend #3732

[juliushaertl]

Reordered them a bit on what I would consider the order of priority, hope that makes sense ;)