fix: Remove invalid session tokens + Login using the Return key

frontend/src/components/app.tsx

@@ -44,6 +44,8 @@ export default class Application extends React.Component<IProps> {
                 this.props.setUser(user);
                 this.props.setAuthenticated(true);
             }).catch(err => {
+                // The token is invalid, so we should remove it
+                removeSessionToken(window);
                 this.props.setAuthenticated(false);
             });
         }
@@ -137,6 +139,7 @@ export default class Application extends React.Component<IProps> {
     }
 
     getTopTenLearners = (): Promise<TopTen[]> => {
+        // TODO: Deprecate?
         const id = this.props.user.classId;
         return new Promise((res, rej) => {
             fetch(`${BACKEND_URL}/api/class/${id}/topTen`, {