R1:1b325a0a899a

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