Bugfix: Visible separator between OcSidebarNav items

We have added a small line between OcSidebarNav items in the `active` 
and `hover` state to visually make them better differentiable.

https://github.com/owncloud/owncloud-design-system/issues/1387
https://github.com/owncloud/owncloud-design-system/pull/1390
