Add support to Files for file storage formats, to support encryption-at-rest

Summary:
Ref T11140. When reading and writing files, we optionally apply a "storage format" to them.

The default format is "raw", which means we just store the raw data.

This change modularizes formats and adds a "rot13" format, which proves formatting works and is testable. In the future, I'll add real encryption formats.

Test Plan:
  - Added unit tests.
  - Viewed files in web UI.
  - Changed a file's format to rot13, saw the data get rotated on display.
  - Set default format to rot13:
    - Uploaded a small file, verified data was stored as rot13.
    - Uploaded a large file, verified metadata was stored as "raw" (just a type, no actual data) and blob data was stored as rot13.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11140

Differential Revision: https://secure.phabricator.com/D16122

src/applications/files/engine/PhabricatorChunkedFileStorageEngine.php

@@ -174,7 +174,7 @@ final class PhabricatorChunkedFileStorageEngine
     return (4 * 1024 * 1024);
   }
 
-  public function getFileDataIterator(PhabricatorFile $file, $begin, $end) {
+  public function getRawFileDataIterator(PhabricatorFile $file, $begin, $end) {
     $chunks = id(new PhabricatorFileChunkQuery())
       ->setViewer(PhabricatorUser::getOmnipotentUser())
       ->withChunkHandles(array($file->getStorageHandle()))