[session] move context manager logic in the security context manager
The `init_security` and `reset_security` method have no other user that the context manager itself. Having dedicated public function is not useful.
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html