Support HTML5 / Javascript chunked file uploads

Summary:
Ref T7149. This adds chunking support to drag-and-drop uploads. It never activates right now unless you hack things up, since the chunk engine is still hard-coded as disabled.

The overall approach is the same as `arc upload` in D12061, with some slight changes to the API return values to avoid a few extra HTTP calls.

Test Plan:
  - Enabled chunk engine.
  - Uploaded some READMEs in a bunch of tiny 32 byte chunks.
  - Worked out of the box in Safari, Chrome, Firefox.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7149

Differential Revision: https://secure.phabricator.com/D12066

src/applications/files/engine/PhabricatorChunkedFileStorageEngine.php

@@ -162,7 +162,7 @@ final class PhabricatorChunkedFileStorageEngine
     return false;
   }
 
-  private function getChunkSize() {
+  public function getChunkSize() {
     // TODO: This is an artificially small size to make it easier to
     // test chunking.
     return 32;