Restore computeNotifications

Summary:
Restore the logic for setting and computing notifications that was
partially removed with D10300838.

Reviewed By: jknoxville

Differential Revision: D10361547

fbshipit-source-id: 4d229d5f4dbeda3139463e1c348909b9c5dba66f

src/reducers/notifications.js

@@ -4,17 +4,7 @@
  * LICENSE file in the root directory of this source tree.
  * @format
  */
-import type {Node} from 'react';
-
-export type Notification = {|
-  id: string,
-  title: string,
-  message: Node,
-  severity: 'warning' | 'error',
-  timestamp?: number,
-  category?: string,
-  action?: string,
-|};
+import type {Notification} from '../plugin';
 
 export type PluginNotification = {|
   notification: Notification,