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

Set the envelop-from from reply-to if smtp.from is not defined in
configuration (patch D. Haag)

To retrieve the min. score, max. score and the cut value, double check
if the resource is really based on QtiWorks otherwise fallback to the
old configuration. Fix an issue in statistics where the max. score and
cut value are retrieve from the wrong place. Cache the min. score and
max. score in the results list of the course assessed identities to
prevent reloading the values for every user.