[py3] ensure AjaxController return bytes publish() must return bytes, when the @ajaxfunc decorated method return None, this raised: TypeError: You cannot set Response.body to a text object (use Response.text)

[py3] Let InstanceCommand.run_arg() always return an int The caller of this method (which is method "run_args") does `max(status, self.run_arg(appid))` and run_arg might return None if the underlying <cmdname>_instance method returns None. On python3 max(1, None) breaks with "TypeError: unorderable types: int() > NoneType()". So we enforce run_arg() method to return an integer even if the underlying method returns None. This fixes call of "cubicweb-ctl i18ninstance" with Python3.

[test] Use cubicweb-file < 2.0.0 From version 2.0.0, cubicweb-file moved to new-style layout (Python package) but we still expect it to be an old-style cube in CubicWeb's tests (WebconfigTC in web/test/unittest_webconfig.py).

[py3] Decode subprocess stdout in toolsutils.show_diffs() On Python 3, subprocess' pipes are bytes stream so we must decode them before printing. Without this change, diff of configuration files (e.g. all-in-one.conf as displayed upon instance upgrade) will rendered as a single line on Python 3.