Summary:
Ref T4324.
- Create `Listener` to represent listening clients.
- Create `ListenerList` to represent the current list of clients.
- Create `Logfile` to handle logging.
Test Plan: Clicked "Send Test Notification", verified logs, status and notifications all work correctly.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T4324
Differential Revision: https://secure.phabricator.com/D8256
