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/files/controller/PhabricatorFileListController.php

@@ -2,19 +2,17 @@
 
 final class PhabricatorFileListController extends PhabricatorFileController {
 
-  private $key;
-
   public function shouldAllowPublic() {
     return true;
   }
 
-  public function willProcessRequest(array $data) {
-    $this->key = idx($data, 'key');
+  public function isGlobalDragAndDropUploadEnabled() {
+    return true;
   }
 
-  public function processRequest() {
+  public function handleRequest(AphrontRequest $request) {
     $controller = id(new PhabricatorApplicationSearchController())
-      ->setQueryKey($this->key)
+      ->setQueryKey($request->getURIData('key'))
       ->setSearchEngine(new PhabricatorFileSearchEngine())
       ->setNavigation($this->buildSideNavView());
 

src/applications/files/controller/PhabricatorFileUploadController.php

@@ -2,8 +2,11 @@
 
 final class PhabricatorFileUploadController extends PhabricatorFileController {
 
-  public function processRequest() {
-    $request = $this->getRequest();
+  public function isGlobalDragAndDropUploadEnabled() {
+    return true;
+  }
+
+  public function handleRequest(AphrontRequest $request) {
     $viewer = $request->getUser();
 
     $file = PhabricatorFile::initializeNewFile();