Move clipboard_button helper to ButtonHelper module

app/helpers/button_helper.rb

 module ButtonHelper
+  def clipboard_button
+    content_tag :button,
+      icon('clipboard'),
+      class: 'btn btn-xs btn-clipboard js-clipboard-trigger',
+      type: :button
+  end
+
   def http_clone_button(project)
     klass = 'btn'
     klass << ' active'      if default_clone_protocol == 'http'

app/helpers/clipboard_helper.rb

-module ClipboardHelper
-  def clipboard_button
-    content_tag :button,
-      icon('clipboard'),
-      class: 'btn btn-xs btn-clipboard js-clipboard-trigger',
-      type: :button
-  end
-end