Show has_part descendants for ontology terms

[hlapp]

At present, only the terms to which the currently shown (anatomy) ontology term has a part_of relationship are shown, which means that in the containment (or partonomy) hierarchy, it is only showing the containing elements, but not the contained ones. Aside from the possibility of using the API (which for this property is also not straightforward at present), it would be quite helpful to have them displayed in the UI as well.

cc @Lucytheanimefan