Provide vertical margins around login buttons
Summary: Ref T1536. If you only have button-based logins, the new login screen looks weird.
Test Plan:
Before
{F46725}
After
{F46726}
Reviewers: chad, jamesr
Reviewed By: chad
CC: aran
Maniphest Tasks: T1536
Differential Revision: https://secure.phabricator.com/D6225