OR-674 add a new event system for usernotification.
On new usernotification:created||updated, send back the notification to
users who are in targets.
OR-674 add a getManagers method which will be reworked later
OR-674 rework wsserver/notification/usernotifications to split categories of usernotification for which we send to all and for which we only send to managers
Merge pull request #507 in OR/rse from ~SLEMAISTRE/rse:OR-674 to master

* commit '7fdd939dbd261976fa8f4c8d1c9e3b3b5e7cda60':
  OR-674 rework wsserver/notification/usernotifications to split categories of usernotification for which we send to all and for which we only send to managers
  OR-674 add a getManagers method which will be reworked later
  OR-674 add a new event system for usernotification. On new usernotification:created||updated, send back the notification to users who are in targets.