cache served microformats2 responses

[snarfed]

...with a short expiration. ideally include the cache expiration in the headers and/or content itself.

alternatively, serve them from the stored response JSON in the datastore. drawback is that we don't get updated content.

[snarfed]

when we hit the G+ daily API limit, we can't fetch comments or likes in mf2 handlers, which results in ugly half-rendered comments. caching would help a bit; serving from stored responses would fix it entirely.

from discussion in IRC. cc @tantek.

[snarfed]

done, yay. oldest issue we've closed in a long time. :P

oh and for the record, serving from stored responses would be tricky since we always want to serve the current silo response, or 404/410 if it's missing, etc. this should usually still work with stored responses, since we update them during poll if they change, but only every poll period, so they could be stale, and we also may often miss updating them entirely right now since we don't see responses past the first page. that's #657.