Add a "Send Test Notification" button to make testing the server easier

Summary: Ref T4324. Currently, it's a bit of a pain to send yourself notifications, and often involves multiple browsers. Instead, add a button to send them.

Test Plan: {F114495}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4324

Differential Revision: https://secure.phabricator.com/D8255