Add a link to sign up to email alerts

This changes the sub-topic view to include a link to the email signup
form.
Use a feature flag to hide the email signup link

This adds a feature flag to restrict the display of the email signup
link in production environments.

We'll override this on preview to display the link by overriding this
initializer in the deploy scripts.
New layout for email alerts and latest feed link

This restyles the previously added email alerts link based on feedback
and suggestions from @futurefabric.

I've also added a link to the latest changes feed here.
Merge pull request #61 from alphagov/add-email-signup-link

Add a link to the latest feed and email alert sign up form
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
Only render timestamp if one is available.

The bump to the API gem provides an example
document without a timestamp to prove that the
code continues to function.
Merge pull request #63 from alphagov/handle-missing-timestamps-in-latest

Only render timestamp for latest documents if one is available.
Tidy up the styling of the email signup form

Working with @futurefabric, this commit tidies up the styling of the
email alert subscription form and adds new copy.

As we've removed the sub-topic title from the copy, I've removed the
associated Cucumber step that checks for it.
Expose separate parent and child slugs on Subcategory

We need both of these to build urls for some of the routes in this
application.

By splitting the slug on slashes, the first value will be the
parent, and the second will be the child.
Add breadcrumps to the email signup page

This uses the dummy artefact pattern to add breadcrumbs to the email
subscription page for a sub-topic, linking back to the topic and
sub-topic, so that users can find their way back to the sub-topic if
they do not wish to subscribe to email alerts.
Merge pull request #64 from alphagov/tidy-up-subscription-page

Tidy up the email alert subscription page