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