Homepage sprucing, spacing normalization

Summary: Cleans up the homepage a little bit. Removes the subheaders and buttons, links the panel header, and adds an icon for further hinting. Also aligned things up to the common 16px gutter.

Test Plan: Tested home, differential, and maniphest. Screenshotted changes

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8034