Improve caching of static resources

Right now, with static resources, we either have to serve with an Expires: header (which means that if the resource is changed by a plugin modification or an upgrade, browsers won't know), or we have to use Last-Modified/eTag (which means that the browser has to do a request/response round trip for every resource in a page, even if it doesn't need to be served.

By giving each resource a dynamic URL that changes when the resource changes, and then setting them to expire in the distant future, we can get the best of both worlds.

Candidates for this technique:

• CSS stylesheets
• Javascript files (this could mean we could do away with the combined Javascript servlet!)
• Plugin downloadable resources