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/home/controller/PhabricatorHomeMainController.php

@@ -2,19 +2,18 @@
 
 final class PhabricatorHomeMainController extends PhabricatorHomeController {
 
-  private $only;
   private $minipanels = array();
 
   public function shouldAllowPublic() {
     return true;
   }
 
-  public function willProcessRequest(array $data) {
-    $this->only = idx($data, 'only');
+  public function isGlobalDragAndDropUploadEnabled() {
+    return true;
   }
 
-  public function processRequest() {
-    $user = $this->getRequest()->getUser();
+  public function handleRequest(AphrontRequest $request) {
+    $user = $request->getUser();
 
     $dashboard = PhabricatorDashboardInstall::getDashboard(
       $user,
@@ -42,7 +41,7 @@ final class PhabricatorHomeMainController extends PhabricatorHomeController {
       $content = $this->buildMainResponse($projects);
     }
 
-    if (!$this->only) {
+    if (!$request->getURIData('only')) {
       $nav = $this->buildNav();
       $nav->appendChild(
         array(