Add a "Download Raw Diff" action to Differential

Summary: this lets people download diffs easily in case arc export, arc patch, etc isn't to their liking or whatever.

Test Plan: "Download Raw Diff" a few times with various things toggled in the "Revision Update History" widget

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1294

Differential Revision: https://secure.phabricator.com/D2855