Allow each finder to have a 'signup' page

Route: `/:finder_slug/email-signup`

Users visiting this page see copy which comes from the artefact:

 * name: used for page title and initial summary paragraph.
 * body: Used for the second paragraph, free-form.

This allows users to sign up for alerts, using details present in the
artefact for display.
Re-shuffle css a bit

Some styles were too tightly coupled to classes so I have broken them out of those parents.
This will need a rebase against some work I've done on Andy's branch as there are overlapping changes for refactoring css there.
Remove extraneous .gitkeep
Introduce email alert subscription workflow

This is the basis of the workflow for fetching topic urls from govuk_delivery api. It would be nice to remove the reliance on the Finder so heavily however as an MVP this should be sufficient. We may require the finder later on.

We have tried to decouple the EmailAlertSignupAPI wrapper from the Finder as much as possible so if we do wish to change this later it should be much simpler.
Hide email signups from robots for unreleased finders
Use description from details hash
Merge pull request #77 from alphagov/finder-email-signups

Allow users to subscribe to email alerts for a finder