Replaced helmet, cors & policy w/ local implem.

The used functionality of helmet, cors & referrer-policy has been
defined in setupCors and setupHeaders.

package.json

@@ -18,16 +18,12 @@
     "canvas-confetti": "^1.2.0",
     "chart.js": "^2.9.3",
     "clean-webpack-plugin": "^3.0.0",
-    "compression": "^1.7.4",
     "connect-mongo": "^3.2.0",
-    "cors": "^2.8.5",
     "express": "^4.17.1",
     "express-session": "^1.17.0",
     "extract-text-webpack-plugin": "^3.0.2",
-    "feature-policy": "^0.4.0",
-    "helmet": "^3.21.2",
     "moment": "^2.24.0",
-    "mongoose": "^5.8.7",
+    "mongoose": "^5.10.9",
     "node-fetch": "^2.6.0",
     "node-sass": "^4.13.0",
     "node-schedule": "^1.3.2",
@@ -35,7 +31,6 @@
     "passport-local": "^1.0.0",
     "passport-local-mongoose": "^6.0.1",
     "qrcode": "^1.4.4",
-    "referrer-policy": "^1.2.0",
     "socket.io": "^2.3.0",
     "socket.io-client": "^2.3.0",
     "vue": "~2.6",