Rename 'facet' to 'filter'

Ahead of adding in some filter options that are not facets i'm renaming the class "facet" to "filter" so that these new non-facet options can share styles without being confusingly classed.
Remove 'facet-menu' class and update cucumber helper
Add checkbox-filter class

- Scope checkbox filter style to them only ahead of breaking this css up a bit
- Add the class checkbox-filter to checkbox filters
Remove class chaining to 'filter'
Rename 'radio-facet' to 'radio-filter'

Things that are styled as filters do not have to be informed by facets.
Remove select styling as it is not used
Improve comments around trickier css
Move .toggle css out of .controls scope

To flatten out the nesting a bit
Move CSS for .closed scrollbars and toggle

- move .closed and scrollbars to the top of the .checkbox-filter to improve readability
- move all toggle styling to be a child of same @include ie-not-lte(7) for readability
Rename 'facet' to 'filter' in JavaScript
Merge pull request #25 from alphagov/rename-facet-to-filter

Rename 'facet' to 'filter' in frontend code