Apps are built on the extensions system, but the extensions system really shouldn't know about apps, and the extensions team isn't maintaining the apps platform.  The fact that much of the apps platform is implemented in extensions directories leads to ambiguous ownership and technical debt.  We should move the apps code to apps-specific directories, and ideally limit (though likely not fully eliminate) extensions knowledge of apps.

Design doc:
https://docs.google.com/document/d/1KcJKXxYdFF9c29Uef_BClWZJPrGBjrhgYcOrLjeUxjo/edit#

(Internal only for now)