WebSocket: Rework backend-for-frontend

[sroidl]

trafficstars

Currently we only support updating the clients by polling. This is done via websockets, but we do not leverage the full potential of websockets yet.

We should:

• Push updates about build summaries,
• build details
• build outputs (already working)

from the server to all clients in order to assure updating clients in a timely fashion and to avoid unnecessary traffic cause by polling.