Get rid of BUILD_ID

[arunoda]

BUILD_ID is a short-term solution to make our assets immutable and properly cacheable.

But it's a pretty good idea to use individual hashes of each files rather a common BUILD_ID which is regenerated per each deployment.

When doing this, we can't have a hash for JSON pages. That's because, we need know hashes for all of the links in the client side. Which is not good. So, we'll use simple etags bases caching method for that.