Realtime Websocket API for general events

[markusressel]

Is your feature request related to a problem? Please describe.
The purpose of this backend is to provide realtime interaction. When users create, modify or delete documents, sections or resources, there is no way of letting clients know about this. This leads to inconsistencies between the server and the client which can cause errors like f.ex. trying to open an already renamed, moved or deleted document.

Describe the solution you'd like
Provide a realtime communication channel between the server and the client, similar to how the document sync works, but for general purpose events. This API should be websocket based and should replace the REST api.

[markusressel]

While this should help with updating the client side more quickly and using delta information, things like auf bad network connection can still cause events to get lost. A mechanism to fully sync the client with the server should still be possible.