added user model

frontend/src/model/user.js

+export default class User {
+    constructor(id_user, id_role, login, avatar, lastname, firstname, mail) {
+        this.id_user = id_user;
+        this.id_role = id_role;
+        this.login = login;
+        this.avatar = avatar;
+        this.lastname = lastname;
+        this.firstname = firstname;
+        this.mail = mail;
+    }
+  }
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