Remove the 'latest_subscribe' partial

This removes the 'latest_subscribe' partial and moves its contents into
the 'subcategory' layout, the place where the partial was previously 
invoked.

It doesn't feel like splitting this out into a partial gives us that 
much clarity, and it complicates showing the links on a conditional 
basis.
Hide the latest feed link on the latest feed

This changes the subcategory layout template to hide the link to the 
latest feed when the latest feed is already being viewed.

This also adds an integration test for the behaviour.
Merge pull request #62 from alphagov/hide-latest-link-on-latest-view

Hide the latest feed link when viewing the latest feed