Make copy button in collapsed sidebar work.

app/assets/stylesheets/pages/issuable.scss

@@ -235,6 +235,19 @@
         display: block;
         margin-top: 0;
       }
+
+      .btn-clipboard {
+        border: none;
+
+        &:hover {
+          background: transparent;
+        }
+
+        i {
+          color: #999999;
+        }
+      }
+
     }
 
   }

app/views/shared/issuable/_sidebar.html.haml

@@ -118,7 +118,7 @@
       - project_ref = cross_project_reference(@project, issuable)
       .block.project-reference
         .sidebar-collapsed-icon
-          = icon('clipboard')
+          = clipboard_button(clipboard_text: project_ref)
         .title
         .cross-project-reference
           %span