Improve homepage buttons for new users

Summary:
Show application names, then a human-readable description of what they're for.

Eventually we'll have better help / tutorial / onboarding / etc systems too.

Test Plan: See screenshot.

Reviewers: btrahan, mgummelt

Reviewed By: btrahan

CC: aran, davidreuss

Differential Revision: https://secure.phabricator.com/D2075