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:

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

Test Plan:

• Created a dashboard panel on a project.
• Clicked it, saw it render.
• Made it the default panel, viewed project default screen, saw dashboard.
• Disabled every panel I could, still saw reasonable behavior (this is silly anyway).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11957

Differential Revision: https://secure.phabricator.com/D17255