BETTERZON-75: User registration API endpoint (#34)

* BETTERZON-75: Adding backend functions to enable user registration

* BETTERZON-75: Adding regex to check email and username

Backend/src/models/users/user.interface.ts

@@ -0,0 +1,9 @@
+export interface User {
+    user_id: number;
+    username: string;
+    email: string;
+    password_hash: string;
+    hashing_salt: string;
+    registration_date: Date;
+    last_login_date: Date;
+}