As a user, I want to get live notifications if there is a new item in my radar

[valfink]

"I need to manually check my Radar for new items. Can't you give me some notifications instead?"

• backend
  • upon adding or editing a food item, check all active Radars if it lies within their ranges
  • send out a WebSocket notification to every Radar's user
  • add RadarNotification model
  • possibly change Chat WebSocket notification structure accordingly
  • best case: check asynchronously so the donator of the new food item doesn't have to wait for all Radars to be checked
• frontend
  • establish WebSocket connection right upon login, so users can get a notification anywhere within the app
  • possibly display Chat notifications anywhere within the app too
• unit and integration tests