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:
{F379292}

{F379293}

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

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

src/applications/phid/type/PhabricatorPHIDType.php

@@ -31,6 +31,13 @@ abstract class PhabricatorPHIDType {
   }
 
   public function getTypeIcon() {
+    // Default to the application icon if the type doesn't specify one.
+    $application_class = $this->getPHIDTypeApplicationClass();
+    if ($application_class) {
+      $application = newv($application_class, array());
+      return $application->getFontIcon();
+    }
+
     return null;
   }