Make it much easier to add one-off event listeners

Summary:
Ref T4657. Right now, you have to muck with `events.listeners` to install listeners. Instead, automatically install all subclasses of AutoEventListener.

Primarily, this makes it easier to resolve requests with "drop this file in `src/extensions/`, no warranty", which seems to have worked well so far in resolving things like custom remarkup rules, etc.

Test Plan:
  - Added such a listener, had it autoregister.
  - Clicked around and saw the effects of normal listeners.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4657

Differential Revision: https://secure.phabricator.com/D9262