Tighten up margins for email alerts on mobile

https://www.pivotaltracker.com/story/show/81950824

This changes the stylesheet for email alerts to reduce the margins between elements on the email alert subscribe page, when viewed on mobile.

FAO @edds 

Before:
<img src="https://cloud.githubusercontent.com/assets/71922/5142067/dee6a252-7175-11e4-99a1-7977e05a8184.png" width="320px;">

After:
<img src="https://cloud.githubusercontent.com/assets/71922/5142069/e4757a68-7175-11e4-9269-83baa9385ec9.png" width="320px;">