Removing all "any" from back.

To do this, I used generic-type-guard package which generates both an interface AND a valid type guard from code.
With this, we are 100% sure that the messages we receive are validated at runtime!
The client cannot pass us an object that is invalid! \o/

back/src/Model/Websocket/UserInGroupInterface.ts

@@ -0,0 +1,5 @@
+export interface UserInGroupInterface {
+    userId: string,
+    name: string,
+    initiator: boolean
+}