Real-time communication with frontend

- Frontend shows heatmap of most visit places
- Maximum accuracy can now be set
- Fix bug where battery chart filtered values wrongly

frontend/package.json

@@ -30,6 +30,7 @@
     "moment": "^2.29.1",
     "ng2-charts": "^2.4.2",
     "ngx-mapbox-gl": "^4.8.1",
+    "ngx-mqtt": "^7.0.14",
     "rxjs": "~6.6.0",
     "tslib": "^2.0.0",
     "zone.js": "~0.10.2"