Add .button:link to override a:link.
Add .button:link to override a:link.

Otherwise, the text on <a class='button'> isn't white when a:link
specifies a link colour.

alphagov/govuk_frontend_toolkit,ministryofjustice/govuk_frontend_toolkit,TFOH/govuk_frontend_toolkit,alphagov/govuk_frontend_toolkit,TFOH/govuk_frontend_toolkit,ministryofjustice/govuk_frontend_toolkit,quis/govuk_frontend_toolkit,alphagov/govuk_frontend_toolkit,TFOH/govuk_frontend_toolkit,quis/govuk_frontend_toolkit,quis/govuk_frontend_toolkit,alphagov/govuk_frontend_toolkit,quis/govuk_frontend_toolkit,TFOH/govuk_frontend_toolkit