Merge branch 'issuable-sidebar' into 'master'

Issuable sidebar tweaks

- Move cross-project reference down 
- ~~Fix "Notifications" label color~~ Reverted
- ~~Use regularly colored button for "Subscribe"/"Unsubscribe"~~ Reverted

Before:

![Screen_Shot_2015-12-15_at_17.38.08](/uploads/d4eeab44bb21fca2fe9ebe620ed9e060/Screen_Shot_2015-12-15_at_17.38.08.png)

After:

(Only the moved cross-project reference is still relevant, other changes have been reverted)

![Screen_Shot_2015-12-15_at_17.38.23](/uploads/b61aed3a0884c986785a5b09af8dbb25/Screen_Shot_2015-12-15_at_17.38.23.png)

cc @skyruler 

cc @rspeicher 8.3 please!

See merge request !2105

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

 .issuable-sidebar.issuable-affix
   = form_for [@project.namespace.becomes(Namespace), @project, issuable], remote: true, html: {class: 'issuable-context-form inline-update js-issuable-update'} do |f|
-    .block
-      .title
-        Cross-project reference
-      .cross-project-reference
-        %span#cross-project-reference
-          = cross_project_reference(@project, issuable)
-        = clipboard_button(clipboard_target: 'span#cross-project-reference')
-
     .block.assignee
       .title
         %label
@@ -62,6 +54,14 @@
           = f.collection_select :label_ids, issuable.project.labels.all, :id, :name,
             { selected: issuable.label_ids }, multiple: true, class: 'select2 js-select2', data: { placeholder: "Select labels" }
 
+    .block
+      .title
+        Cross-project reference
+      .cross-project-reference
+        %span#cross-project-reference
+          = cross_project_reference(@project, issuable)
+        = clipboard_button(clipboard_target: 'span#cross-project-reference')
+
     = render "shared/issuable/participants", participants: issuable.participants(current_user)
 
     - if current_user