Add styles for back links
Add styles for back links

- Use generated content for left pointing black arrow
- Improve arrow shape in Firefox by using rgba
- Fallback to an image for IE8 and below

alphagov/govuk_elements,joelanman/govuk_elements,alphagov/govuk_elements,TFOH/govuk_elements,gavboulton/govuk_elements,gavboulton/govuk_elements,alphagov/govuk_elements,joelanman/govuk_elements,gavboulton/govuk_elements,TFOH/govuk_elements,gavboulton/govuk_elements,alphagov/govuk_elements