R1:0bd8374c634d

Add Content-Length header to Aphront file responses.

Summary:
Provide a Content-Length header so that browsers can estimate time
remaining for file downloads.

Test Plan: Tested on our local phabricator install.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D6107