Make comment preview button white

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/views/projects/notes/_form.html.haml

@@ -7,7 +7,7 @@
   = f.hidden_field :noteable_type
 
   .note_text_and_preview.js-toggler-container
-    %a.btn.btn-primary.js-note-preview-button.js-toggler-target.turn-off{ href: "javascript:;", data: {url: preview_project_notes_path(@project)} }
+    %a.btn.js-note-preview-button.js-toggler-target.turn-off{ href: "javascript:;", data: {url: preview_project_notes_path(@project)} }
       %i.icon-eye-open
       Preview
     %a.btn.btn-primary.js-note-edit-button.js-toggler-target.turn-off{ href: "javascript:;" }