Cache homepage API for 6 hours (bug 1088362)

https://bugzilla.mozilla.org/show_bug.cgi?id=1088362
I had the exact same thought, but I decided not to do it in this PR because categories (and search) are automatically fed, so users might expect their apps to show up quickly after publishing them ? Not sure. Homepage is the biggest priority, so maybe file a (single) bug for the others ?