Factorise `ButtonHelper#clipboard_button` and `ButtonHelper#clipboard_button_with_class`

There are so much duplication between these two methods, that's ridiculous:

  def clipboard_button(data = {})
    content_tag :button,
      icon('clipboard'),
      class: "btn btn-clipboard",
      data: data,
      type: :button
  end

  def clipboard_button_with_class(data = {}, css_class: 'btn-clipboard')
    content_tag :button,
      icon('clipboard'),
      class: "btn #{css_class}",
      data: data,
      type: :button
  end