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