Restyle username/password login for reduced hideousness

Summary: Ref T1536. Error state is a bit gross but we need to sort that out in general.

Test Plan:
{F46549}

{F46550}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6208