FEATURE: Allow denying creation of Asset Collections on root level

[lorenzulrich]

HierarchicalAssetCollection privileges can be used to restrict editors in a multi-site installation to a configured Asset Collection (and their children).

However, given the "manage asset collections" privilege, editors can still create an Asset Collection on root level, which in this case is not a wanted behaviour because users with permissions to other sites will see these collections.

[Sebobo]

Would you say, we should introduce the equivalent of the ReadHierarchicalAssetCollectionPrivilege for creating collections?

[lorenzulrich]

@Sebobo Yes, that's what I would suggest.