Check ?rev when serving static files

[ianb]

We encountered an issue where an old JavaScript file got cached under a new revision, due to race conditions in the deployment (e.g., someone gets a new app page, but when getting the JavaScript they get routed to an old app server).

I think a simple and fairly safe fix would be to check the ?rev, and if it doesn't match what we expect, then don't let the response be cached. Then everything will mostly work during the deployment (old files will probably work), but old files won't get cached.