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.