Browsers should cache editions for a shorter time period than Fastly

Currently we set Cache-Control: max-age=31536000 so that Fastly caches uploads from LTD Mason for a year on its POPs. This has the side-effect of also having browsers potentially cache documentation on the client for up to a year. In practice, browsers churn through their cache space more quickly, but I've noticed that Safari has no cap on its cache space, and therefore can hold onto pages for a long time.

The solution is to set a Surrogate-Control max age to 1 year, and have Cache-Control: max-age=0, private, must-revalidate. This will be done on LTD Keeper during the copy phase of a build into an edition (since it is reasonable for a client to cache a build forever), but then give us the flexibility to update an edition instantly.

In the future we may want a more nuanced solution where CSS and JavaScript, for example, are cached longer on the browser.