Disable auto-zoom on mobile form UIs

Summary: Chrome and Safari both zoom in on form (input, select, textarea) when it thinks the text is too small (less than 16px... which is huge). This turns user-scalable off. The only drawback is double-tap to zoom will be disabled as well, but given we already responsively design, I don't think thats an issue.

Test Plan: iOS simulator on secure and local test instances. Click on an input, no longer see UI zoom in.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D17714