Configuration to define an allowed domain for recipients and if
recipient not in the domain, the from of the mail fallback to the email
address defined by "fromemail"

Use the same protection in D&D match interaction against pushing twice
the same source element in the same target

Subscribe to mails notifications in an asynchronous way to prevent an
error to break an important process and as the subscription can do a
commit, prevent to break some "select ... for update" locking

Fix the NPE if a single page is used to structure tests or assessment
course elements