Refactor users management (#8)

* Update users backend interface signatures

src/main.rs

@@ -61,7 +61,9 @@ async fn main() -> std::io::Result<()> {
             .expect("Failed to create initial user!");
 
         // Set default admin password
-        users.change_user_password(&default_admin.uid, DEFAULT_ADMIN_PASSWORD, true);
+        users
+            .change_user_password(&default_admin.uid, DEFAULT_ADMIN_PASSWORD, true)
+            .expect("Failed to change default admin password!");
     }
 
     let users_actor = UsersActor::new(users).start();