[hooks] Change get_user_sessions prototype, necessary for hijacking from other session implementations (e.g. pyramid)

Store user groups and properties as session data * stop retrieving them systematically, only when need, * reimplement session synchronization hooks with some cleanups along the way, * cleanup call to set language: not needed from the base request nor from the server side, only for the web request (on the server side, language is necessary only for notification and such code should set it explicitly). There is still a XXX remaining about one can only "enter" a connection once and this is a problem in some cases. IMO, this restriction could be removed. Closes #13500113.

[skeleton] Remove override_dh_python2 We do not ship files as "data files" anymore. Related to #13001466.

[skeleton] Have setuptools install data files Just add `include_package_data=True` as recommended by `setuptools documentation`_ as this appears to be the simplest way to handle this. Related to #13001466. .. _`setuptools documentation`: \ http://setuptools.readthedocs.io/en/latest/setuptools.html#including-data-files

[skeleton] Update MANIFEST.in following new "package" layout Related to #13001466.

[skeleton] Add python-setuptools to Debian Build-Depends Related to #13001466.