Modularize SearchEngine extensions
Summary:
Ref T9964. ApplicationSearch currently has a bunch of hard-coded if ($object instanceof thing) stuff.
Pull that out so it can live in extensions.
Test Plan:
- Searched by spaces, subscribers, projects.
{F1023921}
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T9964
Differential Revision: https://secure.phabricator.com/D14764