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
Disable CSRF protection for email signup creation

CSRF can't work on GOV.UK frontend applications because cookies are
stripped before the request is served to the user.
Add date filtering facet
Remove all vertical padding on the top and bottom of filters

Going to add this to contained elements instead so elements stack inside grey boxes sensibly
Put vertical whitespace onto form elements
Remove one level of indentation

I'm trying to restructure the css a bit to make it easier to add new input types to it when required. This commit de-nests some of the SCSS to make it easier to read.
Take .filter, .radio-filter, .checkbox-filter and .text-filter out of .filter-form scoping.
Change comment to make it more understandable
remove superfluous 'px' from '0px'
fix indentation
Move some input styles out of desktop only
Correct label and placeholder
Add margin to top of checkbox-container
Refactor Radio Facet

This commit includes the work to move the sentence generation to the
individual radio facet.
Refactor Select Facet

This commit includes the work to move the sentence generation to the
individual select facet.