Remove silent margin classes

* They shouldn’t be extended, a mixin or variable should be used instead
Move margins into shareable variables

* Create `default-label-margin` and `default-horizontal-margin`
Move margin helpers into re-usable mixins

* Use a mixin rather than extending silent classes
Mixins shouldn't default to using !important

* Make !important an option, use it in margin helper classes
* Common usage should be `@include add-top-margin;` which will provide
the expect margins without an important side effect
Merge pull request #20 from alphagov/margin-mixins

Introduce margin mixins