MDL-31560 Messages: Added support for noreply and support users

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.
MDL-31560 Messages: Deprecated generate_email_supportuser

generate_email_supportuser was not creating user which can
be used by messaging system. core_user::get_support_user()
should be used to get support_user record.
MDL-31560 Error: Fixed coding style