Combine ModelAdmin and HierarchyModelAdmin

[GuySartorelli]

After silverstripe/silverstripe-cms#2951 is done, we'll be able to manage any arbitrary hierarchical model class in a tree structure - but each model class must have its own admin section.

There are cases, such as with silverstripe/taxonomy, where related classes should be managed in different ways.
In the case of siverstripe/taxonomy, the TaxonomyTerm class should be managed as a tree, but TaxonomyType should be managed in a gridfield. But they should be in a shared admin section.

Related

• SPIKE Analyse how LeftAndMain and CMSMain are put together and how they could work better silverstripe-cms#2933

Acceptance criteria

• Subclasses of ModelAdmin can declare that a given tab or tabs should be managed as a tree
• ModelAdmin subclasses can have different models in different tabs, some as trees and some as gridfields
• The TaxonomyTerm class is managed as a tree view