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