Add loading state to notification menu.

Summary: Notification menu shows a loading message before the menu is populated with the actual response.

Test Plan: Checked by having the function sleep between getting the response and populating the menu.

Reviewers: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4696
Merge branch 'master' of github.com:bcuccioli/phabricator