Patch-based Realtime Collaboration

[loreanvictor]

This PR provides support for patch-based realtime collaboration, i.e. syncing clients using patches (with the JSONPatch standard format) instead of syncing state whole-sale.

This PR is a work-in-progress, I am mainly opening it as a way to better test the corresponding PR on Apollon, though it will also host the final updates to Apollon standalone for utilising patch-related features provided by said PR.

[loreanvictor]

This is basically ready for review, pending this PR on Apollon, and a corresponding version update to Apollon. Since that PR is not yet marked as ready for review, I won't mark this as ready for review either, though it can be tested and shouldn't require further changes except for the aforementioned Apollon version update.

To test, simply follow instructions outlined in this PR.:

• Clone the branch and install dependencies

  npm i

• Build the app and create diagrams folder:

  npm run build:local
  mkdir diagrams

• Start the server:

  npm start

• Open localhost:8888, create a diagram
• Get a collaboration link by clicking on Share button on top menu, then Collaborate, then copy the browser URL
• Open the URL in another browser (private browsing or different browser)
• You should see each user's selections
• You should see each user's changes
• You should see a user's selection disappearing when their client is closed

re: @matthiaslehnertum if you could take a look at this it'd be nice!