R1:5960b1c8a3cd

Allow menu items to render their own content; make Dashboard items render on-page

Summary:
Ref T11957. When you click a dashboard item, it now sends you to `/<app>/item/view/123/`, which renders the proper crumbs, navigation, etc., with the dashboard as page content.

This works as you'd expect in Projects:

{F2508568}

It's sliiiightly odd in Favorites since we nuke the nav menu, but seems basically fine?

{F2508571}

Test Plan:
- Created a dashboard panel on a project.
- Clicked it, saw it render.