Implement "Cache, then Network" strategy

[jeffposnick]

As described at http://jakearchibald.com/2014/offline-cookbook/#cache-then-network, this is appropriate for feeds where it's appropriate to initially display cached content, which is then updated with fresh content. We'd initially use this for the social feed (#13) and perhaps the I/O extended feed, but it may end up being used for the session feeds as well.

There are two components to the work: writing a helper function that executes on the page and makes the two requests, and writing a shed handler that responds to the requests.