[request] drop the user argument for set_session I see no code nor test that use this optional argument. removing it help to clean the session code. The set_session function will soon be deprecated anyway (at the same time than the dbapi) related to #2503918

[web-request] handle default language earlier We now read language negotiation header at initialisation time (previously done during ``set_session``). This simplify the set_session code for later reworking. There is no change in behavior, the user selected language continue to overwrite the one picked from the header/default when applicable. related to #2503918

[etwist] pass the repository to the root resource This is another step toward a cleaner instantiation scheme for the repo. The http test now pass an already created object to the etwist server instead of relying on the config cache. Related to #2249513