This is used by messaging system for sending/receiving message
to/from noreply or support user. message_send api will now use
core_user class to get noreply or support user and then
send/receive message depending on user state.
defaults.
At the moment, each message_provider can specify defaults for
each message output, to say whether that message output should be
used for this type of message:
http://docs.moodle.org/dev/Messaging_2.0#Adding_new_message_type.
If the message provider does not set a default, this change allows
the message output to set the default.