Revert "Use regularly colored button for "Subscribe"/"Unsubscribe""

This reverts commit c1db8d15.

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

@@ -70,7 +70,7 @@
         .title
           %label Notifications
         - subscribtion_status = subscribed ? 'subscribed' : 'unsubscribed'
-        %button.btn.btn-block.subscribe-button{:type => 'button'}
+        %button.btn.btn-block.btn-gray.subscribe-button{:type => 'button'}
           %span= subscribed ? 'Unsubscribe' : 'Subscribe'
         .subscription-status{data: {status: subscribtion_status}}
           .unsubscribed{class: ( 'hidden' if subscribed )}