Warn developers when files in /public exceed 10MB

[rglover]

The ideal pattern for public files is to keep it light and to only what's needed for the UI. Anything > 10MB is likely a file that should be moved off to a CDN (e.g., S3/CloudFront).

There's also a built-in payload limit for Joystick apps deployed with Push, so this foreshadows that while encouraging solid practices.

Do it at the joystick.js/node level so it's more of a nag. This way it will actually be handled and not ignored till the last minute.