Be able to specify the wiki root as template provider visibility/creation restriction

Currently, template providers use local space references to specify visibility or creation restrictions and this results in the impossibility to specify a restriction for the wiki root (I.e. top level page) because you have no way to express it.

One suggestion from lucaa was to use type-prefixed entity references (i.e. wiki:wikiName, space:Some.Space, etc.) when storing the restrictions. This would also require a data migration to the new format (for the existing restrictions stored as local space references).