Final cleanup of components before releasing.

Checking / amending:

• print styling - use of @mixins
• link styling - use of @mixins
• add 12px to spacing map and update key values to be the same as px size
• alphabetise backstop.json component list
• triangle @mixin - care card and hero
• class names should use heading and not header
• check macros - each macro should use params and each component should have classes on surrounding element
• check .nhsuk-main-wrapper spacing - should be 48px on desktop and 40px on mobile
• body line-height
• promo component colour variant - grey
• center align element helper class
• update visual tests for all components
• update change log
• use the HTML syntax highlighting in READMEs