This commit groups all mail settings under Server node, splitting into
two categories: Outgoing mail configuration and Incoming mail configuration.
Also, it make maildomain setting available on the Email settings page.
Instead of silently defaulting to SITEID when courseid (coming
from message_send()/\core\message\manager::send_message()) is missing,
now a debugging message is shown to allow developers to fix their
messages to, always, include courseid.
Raw creation of events via message_sent::create() missing other[courseid]
leads to coding exception since now (there shouldn't be any legacy use, as far as
they are always created via create_from_ids() when sending a message.
Updated upgrade.txt notes a little bit, added references the 3.6 final
deprecation issue (MDL-55449) and covered with unit tests.
Remove the HTML tags from any message in the message area when updating
the last message on the contact otherwise the styling breaks and also
allows <script> tags to be added to the page.
Stop the message area header and footer from being rendered if there is
no other user being messaged, otherwise notices are generated and the
messages fail to send.
Adrian Greeve