Use a typeahead for "document types" in global search

Summary:

• Now that we have "browse", this is a much more reasonable control for random sets of things.
• The new explicit search scope selector reduces the need to fiddle with this field manually, too.

Test Plan:

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D12510