Move selectable Sass into header footer stylesheet

Ideally this would be a component with matching markup. However, the
JavaScript to power this and applications currently using this would
need updating to match a component namespace.

As the JavaScript is already served in the header_footer_only JavaScript
file, move the Sass into header_footer_only as well.

Also better scope the Sass during the move so that it is all nested
within the `label.selectable` selector. This will enable switching the
code to a proper component much easier in the future.
Merge pull request #468 from alphagov/selectable-globals

Move selectable Sass into header footer stylesheet
Add Google Analytics tracking for events on the Satisfaction Survey

To better understand how our users interact with the user satisfaction
survey, push some tracking events to Google Analytics. With this
change we'll be able to count the different interactions and get a
better understanding (in the specific) of what's occurring beyond the
case where a user has taken the survey.

It's worth noting that the tracked events are set as non-interactive
events in Google Analytics. A description of this is here:
https://developers.google.com/analytics/devguides/collection/gajs/eventTrackerGuide#non-interaction
Merge pull request #469 from alphagov/add-ga-tracking-for-user-satisfaction-events

Add Google Analytics tracking for events on the Satisfaction Survey