Modernize QuickSearch typeahead

Summary:
Use ClassQuery to find datasources for the quick-search.

Mostly, this allows extensions to add quicksearches.

Test Plan:
using /typeahead/class/, tested several search terms that make sense.
Removed the tag interface from a datasource, which removed it from results.

Reviewers: epriestley, amckinley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D18760