Add user model

frontend/models/user.ts

@@ -0,0 +1,22 @@
+export class UserItem {
+  constructor(
+    public id: number,
+    public username: string,
+    public is_superuser: boolean
+  ) {}
+
+  static valueOf(
+    { id, username, is_superuser }:
+    { id: number, username: string, is_superuser: boolean }
+  ): UserItem {
+    return new UserItem(id, username, is_superuser)
+  }
+
+  toObject(): Object {
+    return {
+      id: this.id,
+      username: this.username,
+      is_superuser: this.is_superuser
+    }
+  }
+}