Use proportional heading classnames

* Remove the value (e.g. -48, -36) from all headings
* Instead, set classnames for headings, ranging from xxlarge to small (this is more flexible, if they ever change).
* Use relative units for margins and padding on all headings and text
* Keep values for margins and padding easy to understand - here we're using a function called `em` which accepts a pixel value and returns a relative value in em `em($px, $base: 19)`, find this in [_helpers.scss](https://github.com/alphagov/govuk_elements/blob/master/public/sass/elements/_helpers.scss#L4)