Tighten up margins for email alerts on mobile

This changes the stylesheet for email alerts to reduce the margins 
between elements on the email alert subscribe page, when viewed on 
mobile.
Merge pull request #65 from alphagov/tidy-up-on-mobile

Tighten up margins for email alerts on mobile
Remove beta header from subtopic show pages.

To quote the ticket:
Now that sub-topics have curated groups, email alerts and a latest feed, and now that they'll be the official (and only) source of content about topics for many different government organisations, we should no longer be denoting them as "beta".

We should still show the "beta" label on topic pages, as we know we still want to iterate these significantly, and the latest changes feed, as not all content have timestamps and change notes.
Merge pull request #66 from alphagov/remove-beta-label

Remove beta header from subtopic show pages.