Refactor the main menu in preparation for a mobile application menu

Summary:
As per discussion, this primes the existing mobile menu / menu button for "phabricator" and "application" menus.

Design here is very rough, I'm just trying to get everything laid in functionally first. It's based on frame_v3.png but missing a lot of touches.

Test Plan:
{F26143}
{F26144}

Reviewers: chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T1960

Differential Revision: https://secure.phabricator.com/D4058