Assess notifications mechanism in new appliance

[jonquet]

Use shall be able to subscribe to ontologies and get notified when :

• new versions is released in AgroPortal
• notes are added
• mappings are added
• others have subscribed to the same ontology

Admins must also be notified when :

• a new user is created (work for now)
• a new ontology is created (work for now)
• a new version of any ontology is being created
• a user just subscribed to an ontology
  others...

[saubin78]

Also check that the "subscribe" button is easily identifiable by the user (in the current Agroportal, I cannot find it. I can only fin the subscription to notes button)