Improve drag-and-drop uploader

Summary:
Make it discoverable, show uploading progress, show file thumbnails, allow you
to remove files, make it a generic form component.

Test Plan:
Uploaded ducks

Reviewed By: tomo
Reviewers: aran, tomo, jungejason, tuomaspelkonen
CC: anjali, aran, epriestley, tomo
Differential Revision: 334

src/applications/files/controller/dropupload/PhabricatorFileDropUploadController.php

@@ -30,12 +30,13 @@ class PhabricatorFileDropUploadController extends PhabricatorFileController {
         'name' => $request->getStr('name'),
       ));
 
+    $view = new AphrontAttachedFileView();
+    $view->setFile($file);
+
     return id(new AphrontAjaxResponse())->setContent(
       array(
-        'name' => $file->getName(),
         'phid' => $file->getPHID(),
-        'size' => $file->getByteSize(),
-        'uri'  => $file->getViewURI(),
+        'html' => $view->render(),
       ));
   }