Enhancement: Spaces context menus

Spaces context menus have been adjusted visibly to match the other available context menus.
Also, the corresponding component has been abstracted in the course of this. This cleans up a lot of (duplicated) code across the spaces views and makes future adjustments easier.

https://github.com/owncloud/web/pull/6659
https://github.com/owncloud/web/issues/6634
