Let the viewer collapse/expand individual files in a diff.

Summary:
Added a dropdown menu button and the keyboard shortcut 'h' to the
web diff view. These hide or show the annotated code display.

Test Plan:
Viewed an example diff that changed a large number of source files
and played around with keyboard shortcuts. Everything seemed to
work as expected.

Reviewers: nh, epriestley

Reviewed By: epriestley

CC: aran, epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D2714