Cache index.html for commonplace projects for a few minutes (bug 1071468)

https://bugzilla.mozilla.org/show_bug.cgi?id=1071468

Those pages are served by us, not the CDN, so we can afford to cache them for a few minutes to help repeat views. If a client wants to bypass the cache (for instance when we push to production) they can hit reload, their browser will send a `If-Modified-Since` or `If-None-Match` and we'll give them the new content.