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
• [ ] combine multiple yjs changes into a single change before they are send to the server
• [ ] 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.