The admin sidebar used to be composed based off of an HTML file

populated using state from a template tag, with template hooks to append

custom HTML to the sections. While there was some degree of

extensibility, it was a bit limited, requiring the consumer to generate

just the right HTML.
This change converts the sidebar to use actions instead. There's now an

action group per group in the sidebar, and an action per child.

Specialized sidebar action renderers take care of rendering with

existing support for the template hook points.
The Manage section has the most specializations. The group renderer

takes care of looking up the counts and Add Item URLs for each child

action, caching the results for quicker lookup. The item renderer then

provides this to a template, based on the cached state (falling back to

direct querying if not in the cache).
With this, extensions can now simply register actions in the Admin UI

sidebar, taking advantage of the built-in rendering and capabilities.

Tested all the links in the admin UI.
Tested fallback Manage item logic when the cache results come up empty.