Disallow back button if username forced

app/assets/app/auth/login/Form.component.js

@@ -65,12 +65,14 @@ export default class AuthLoginForm extends Component {
     loading = false
     error_message = ''
     other_message = ''
+    allow_back = true
 
     watch_username(new_username, old_username) {
         this.btn_disabled = !new_username
     }
 
     back_click() {
+        if ( !this.allow_back ) return;
         this.step_two = false
         this.button_text = 'Next'
     }
@@ -78,6 +80,7 @@ export default class AuthLoginForm extends Component {
     async vue_on_create() {
         const auth_user = await auth_api.get_authenticated_user()
         if ( auth_user ) {
+            this.allow_back = false
             this.username = auth_user
             await this.step_click()
         }