Stop toggle buttons overlapping

Summary:
When there's an AphrontFormToggleButtonsControl with a lot of toggle options,
the rounded shape of the a.toggle buttons overlaps, and the text inside breaks
over multiple lines.

Making them display:block, and floating them left fixes this.

Test Plan: Made my font really big as a test case.

Reviewers: epriestley

Reviewed By: epriestley

CC: bgh, aran, Korvin

Differential Revision: https://secure.phabricator.com/D3529