Quicksand - make things work correctly with global drag and drop upload

Summary: Fixes T7685. This required making the global drag and drop behavior able to "uninstall" itself so to speak, and then it re-installs it self as necessary.

Test Plan:
Did the following all successfully

 - uploaded a file to homepage
 - homepage -> differential -- no way to upload via drag and drop
 - homepage -> differential -> homepage -- uploaded a file
 - homepage -> differential -> browser back button to homepage -- uploaded a file

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7685

Differential Revision: https://secure.phabricator.com/D12534

src/applications/base/controller/PhabricatorController.php

@@ -57,6 +57,10 @@ abstract class PhabricatorController extends AphrontController {
     return false;
   }
 
+  public function isGlobalDragAndDropUploadEnabled() {
+    return false;
+  }
+
   public function willBeginExecution() {
     $request = $this->getRequest();