Add alpha label

Add an alpha label for this ticket: https://trello.com/c/0eHTxG4j/141-add-alpha-label-to-eim-manual
Also add beta-label styles in to header-footer-only.scss
Any reason you're not using the [phase_tag mixin](https://github.com/alphagov/govuk_frontend_toolkit/blob/master/stylesheets/design-patterns/_alpha-beta.scss#L42-L59) from govuk_frontend_toolkit?