AMP now warns and will eventually throw errors when extensions are un-used

[tylr]

AMP created a requirement that any extensions you inject scripts for must actually be implemented on the given page. That means if we use the amp-twitter extension and there are no embedded tweets on the page AMP will create a warning.

This means we'll have to re-evaluate the approach for detecting the usage of and injecting amp extensions onto a given page.

I have some ideas but thought I'd file this issue to start the conversation.