Break some of Aphlict into reasonable classes with sensible responsibilities

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