R1:1eafc2b638ad

Layout, spacing tweaks for new auth.

Summary: Touches a lot of little spacing things here and there, stuck to 4px grid when possible, checked mobile views.

Test Plan: Mobile, Logging In, Multiple Providers.

Reviewers: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D6220