R1:614e826e7bc5

Add fullWidth option to AphrontFormView

Summary: This piggybacks onto device-phone's CSS rules to enable a full width form (for smaller spaces).

Test Plan: Convert New Message dialog to fullWidth.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5924