Enabling Typescript strict mode on the front

This is very important otherwise, a number of useful checks (like nullable objects not propertly checked) are not performed.

See https://dev.to/briwa/how-strict-is-typescript-s-strict-mode-311a

front/tsconfig.json

@@ -7,6 +7,9 @@
     "module": "CommonJS",
     "target": "es5",
     "jsx": "react",
-    "allowJs": true
+    "allowJs": true,
+    "strict": true,                           /* Enable all strict type-checking options. */
+    "noImplicitReturns": true,                /* Report error when not all code paths in function return a value. */
+    "noFallthroughCasesInSwitch": true       /* Report errors for fallthrough cases in switch statement. */
   }
 }