new function: ::content::revision::export_to_filesystem
This change moves functionality from file-storage to acs-content-repository. all application packages should be free of internals, such as the storage type. Storage_type should be handeled in the content-repository package.
We are not yet done, there is still more to do in this respect in the file-storage package.
added constraint cr_items_parent_id_ck to avoid certain simple loops on parent_ids
An item must not be equal to its own parent. Note that this constraint is not guaranteed to avoid all loops; it is still possible to create indirect recursive loops but excludes some real-world problems.