Add missing base 'icon' classes
Add missing base 'icon' classes

Each icon uses the icon base class and a modifier class.

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