Including in this commit:
 - Use language strings from core_enrol to match with enrol_manual
 - Minor update for UI so Custom welcome message text area
   will not be shown if the Send course welcome message is
   set to No
 - enrol_self now using Hook API to send the welcome message
 - enrol_self_plugin::email_welcome_message() has been deprecated
 - Added Behat test to test the welcome message