Support external webpages as modules

[mhkeller]

I'm not super into this approach for module designs since downloading the app is preferable for versioning but if you don't have access to the source code or you're pointing to a text-based company resource or something, this functionality could be useful.

On the external-apps branch, as of 44b18e7 you can specify external as the version number.

TODO It should then load the page with the webview tag.