Add chat

@christopheschwyzer @nicolasschmutz
Added basic logic for chat (form, message-list cmp, websocket channel).

todo:
- [ ] Design (!)
- [ ] Add message on form submit (additionally to websocket binding)
- [ ] VenueNearby form-field: Test radius-search a bit more
- [ ] VenueNearby form-field: Consider geolocation-accuracy? 
- [ ] VenueNearby form-field: Add *failure* UI
- [ ] VenueNearby form-field: Update on location change? 
- [ ] Allow to zoom in on images?