Update etag and field serialization functions to take a HttpRequest.

generate_etag() and serialize_*_field() functions now takes a request
parameter. This requires an update to all callers, and thus is not
backwards-comptaible. It's needed for cases of serialiation that may
have to take into account some detail of the request, such as whether
we're using http or https.

Ran the Djblets and Review Board unit test suites (after appropriate
changes in Review Board).