Offer GitHub Pages-hosted embedded store for testing

To improve onboarding and testing the store:

In case people don't have an easy way to upload HTML files for their store, we could put a template HTML page to GitHub pages. Then users would need to "whitelist" github.com with FastSpring, and that's it.

The package would need to inject the `STORE-ID` and `PRODUCT-ID` into the HTML in that case, though. Might as well inject 100% of the HTML into an empty page in that case.