Email alerts fixes

A set of tweaks and fixes to the new email alerts work. The main thrust of this work was to reinstate atom feed url verification so that we don't fill govdelivery with nonsense alert topics that don't actually exist.

Trello: https://trello.com/c/ZtLwmXAU
User Formats Team Pivotal: https://www.pivotaltracker.com/story/show/59496412