Fix bug 1091939: Pregenerated snippets

Adds the ability to serve pre-generated bundles of snippets that can be shared between clients that will receive the same snippet code, cutting down on request time and moving the heavy lifting of serving snippet content out of the WSGI process.

The feature is behind a waffle flag to allow us to throttle and test easily to see how it affects performance.

I'd hold off on actually merging this until at least after the holidays, but it's good for a review, at least.
Interestingly this returns unicode instead of str. I'd except that you need to `u'_'.join(..)`