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.
Schema of the data
------------------
First take a look at the data schema_ then try to remember that you are browsing
through a heap of data by applying stylesheets to the results of queries. This
site is not a content management system with items placed in folders. It is an
interface to a database which applies a view to retreived data.
.. _schema: ../schema