Add a "hierarchy" type to the Cargo extension

This project aims to introduce hierarchy fields in Cargo MediaWiki extension. This project will enhance the capability of MediaWiki by enabling efficient storage and querying over information which is hierarchical in nature, such as following piece of information :

Chapter Name > Topic > Sub-Topic

Project also involves addition of support for hierarchy fields in Page Forms MediaWiki extension, to ensure easy input from users entering hierarchy data into the wiki as well as to maintain compatibility with frequently used helper pages.