[req,web/request] Move language prefix handling into web request
This changes part of d8830e2bd2e0 by unsetting "lang" attribute of
RequestSessionBase in particular (because it appears to break many downstream
applications). The new implementation relies on a new "build_url_path" method
on RequestSessionBase that is overridden in _CubicWebRequestBase to handle the
language prefix in URL.
As a consequence build_url in RequestSessionBase is not aware of this prefix
anymore (contrary previous implementation from d8830e2bd2e0). Move respected
tests from unittest_req.py into unittest_request.py.
Commenting out part of test_handle_request_no_lang_negotiation_fixed_language
in unittest_application.py because it now fails because of some obscure bug
that cannot be resolved at the moment.
Related to #15743487.