New style for drop-down field trigger

I'm raising this pull request as way of starting a conversation about what we do regarding styling `select` elements. The solution I've presented here implements the design but has a number of problems:
* Requires a modifier class on the field group element. This isn't so bad but is a little redundant. It also doesn't make sense if the field group contains both a select and input field.
* Doesn't cater for when there are multiple select fields within a field group. There is an example of this on the new sign-up form.
* Uses `pointer-events` property, which isn't supported in IE < 11.

@triblondon Any advice for a nicer implementation?
@danskinner @pauloneillft Can we make any comprises on the design?
Comment here or a clearer mixin name would be good.