browser-cache for static content

[fisx]

As of #920, browser caching is universally disabled. This is bad for scalability and for people coming through bad networks. What we want is something more like this:

• cache everything under /static
• do not cache anything else
• every reference to a cached resource should end with [?|&]last_changed=<epoch_time>

we could write a more sophisticated (and much slower) middleware that parses all of html, css, js, and whatever else i forgot, but...

lots of packages on hackage:

• http://hackage.haskell.org/package/wai-middleware-preprocessor
  more for compiling purescript dynamically on delivery
• http://hackage.haskell.org/package/wai-middleware-etag
  almost what we want, but doesn't watch file changes
• http://hackage.haskell.org/package/wai-middleware-static-caching
  probably good for inspiration, but doesn't help with cache busting.
• http://hackage.haskell.org/package/wai-middleware-caching
  only server-side
• http://hackage.haskell.org/package/wai-static-cache
  only server-side
• http://hackage.haskell.org/package/wai-middleware-cache
  only server-side

nothing that does browser cache busting.