Reduce page-header's top margin

* Account for the extra margin from the navbar
Merge pull request #19 from alphagov/page-title-spacing

Correct spacing when using Bootstrap's page-header class
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