Provide ApplicationSearch hooks in Maniphest

Summary: Ref T418. Adds hooks to support customized ApplicationSearch (you can't currently add indexable fields without writing custom code).

Test Plan: Wrote custom code to add an indexable field.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T418

Differential Revision: https://secure.phabricator.com/D7002