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