Add some textures/gradients to crumbs and menu

Summary:
This doesn't lay in everything, but:

• Break the buttons gradient apart into components and rebuild it (along with other gradients) into a single gradient sprite (possible after {D4099}).
• Use the sliced gradient for the crumbs background.
• Use the sliced image for the crumb divider.
• Adds the black/white app sheets, but I'm not generating them quite yet.

Test Plan: {F26537} {F26540}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T1960

Differential Revision: https://secure.phabricator.com/D4100