Adding a label to an issue / MR that already has the label via quick action shows a success message

When generating the success message for adding labels with quick actions in https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/quick_actions/issuable_actions.rb#L262, we should take into account the labels that already exist on the issuable.

If all the labels already exist, we can show a different message such as: "No labels were added"