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