Change: Disable context menu item to get a public link for a space

The context menu item "Create and copy public link to clipboard" didn't
work correctly when used with Spaces: users were always redirected to
the web frontend. This menu item is now disabled, waiting for a
server-side changes.

https://github.com/owncloud/client/issues/11242
https://github.com/owncloud/client/pull/11079
