Enhancement: Batch actions redesign

We've improved the overall look and feel of the top bar batch actions.
This includes the new mechanism that the batch actions show up as well when only one item is selected,
but also includes design changes.

https://github.com/owncloud/web/pull/9346
https://github.com/owncloud/web/issues/9340
https://github.com/owncloud/web/issues/9352
