Align form captions with form elements

Summary: Make captions feel less floaty, align with their input element.

Test Plan:
Visit a few forms with captions, test mobile and desktop layouts

{F236871}

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D10891