[session] move context manager logic in the hook controls context manager
The `init_hooks_mode_categories` and `reset_hooks_mode_categories` have no other
user that the context manager itself. Having dedicated public function is not
useful.
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