Don't prefill "add email address" from GET

Summary: Via HackerOne. I don't think this is a security vulnerability, but it is inconsistent. There's no reason to prefill this, and I think the code was just lazy.

Test Plan:

• Hit this page with ?email=xyz in a GET request, no more prefill.
• Looped the page with bad addresses, appropriate prefill.
• Added an address.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D8458