Cache the HTML page at /pdf/<URL>

The page at via3.hypothes.is/pdf/ is an HTML page containing PDF.js. It should be possible to cache this HTML page in Cloudflare, saving a lot round-trip waiting time for users.

The hazard is that, because we use immutable URLs for all the resources that the HTML page includes (e.g. PDF.js's assets), if these static resources have been changed old copies of the HTML page will still be linking to the old versions of the resource, until the HTML page itself expires from the cache.

Nonetheless, we should hopefully be able to get away with caching the page for some period of time to speed things up. Currently it's not cached at all

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Implementation should be a case of changing the timeout specified in the view from 0 to something.