Warn user before losing his data

Summary: Better solution would be to reload the page for user with valid token and all data he inserted but I guess that we don't have enough infrastructure for this.

Test Plan: Mangle token and send form.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2570