Serving Externals through the registry itself

[nickbalestra]

Currently we serve the externals library defined by the template manifest through an external cdn.
I think would be better if we make templates able to directly serve the needed externals through the registry library itself

[dipunm]

Somewhat related: currently we load the minified "production" packages even when doing a development build. This means that things like PropTypes never warn us during development.

+1!