OO-3213	: implement expiration date in taxonomy, check the managed flag in the user's list of competence, persist preferences of the different tables in taxonomy module

OO-3201: check if the current question is not null to be valid and send a special message if the test is finished

OO-3195: send 507 (insufficient storage) instead of 409 or 419, the WebDAV client give sometimes a better error handling

OO-3193: render the date of the certificate in the excel download of the list of assessed identities

OO-3189: make the link to about page static, remove the invisible logout link from the index page, cannot open several times the about page without closing it, fix a memory leak in high score