Remove default margin on selects

Summary: Fixes T11935. Safari even with appearance: none still assigns margin to selects. Reset margin to 0.

Test Plan: Review selects on Safari, Chrome.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T11935

Differential Revision: https://secure.phabricator.com/D16968