R1:1b8e129145d1

Move message posting to PhabricatorNotificationClient

Summary: Ref T4324. Centralize communication with the notification server. This will probably get less messy eventually.

Test Plan: Posted some messages.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4324

Differential Revision: https://secure.phabricator.com/D8252