Feature/236 instantsearch templates

[tw2113]

I'm feeling like this is going to be a minimal impact overall, as anyone still using the fallback, will likely see zero change with this, and anyone that has customized, is hopefully using this html template tagging method already.

That said, we will definitely still want to let it be known in the changelog that we'd be removing the dependencies with underscores and wp-util, so that they know they may need to re-enqueue