Fix API

[jochenklar]

This PR improves the API in several ways:

• Links are included in the histogram action and the queryset is simplified. This should improve performance with facets in the interface.
• Ordering is removed from the count query.
• The tree_path parameter ensures a trailing slash. This need a migration for the metadata database to add this slash in the tree_path field.
• The datasets and files tables now contain an automated root_id column which is either target_id or id.
• Annotations and caveats are now (manually) prefetched in DatasetViewSet,