Add support for lazily-evaluated BROWSERID_REQUEST_ARGS.

Updates our use of BROWSERID_REQUEST_ARGS to support Django Promises.
This is primarily useful for translating strings in the request args
using lazy translation, as well as reversing URLs using reverse_lazy.
Merge pull request #154 from Osmose/lazy-request-args

Add support for lazily-evaluated BROWSERID_REQUEST_ARGS.