Introduce calendar type choice at system level (Backport of MDL-42932)

Install the calendar type 'test' ( https://github.com/markn86/moodle-calendartype_test ) into the folder calendar/type/test. Visit admin/index.php and install the 'test' calendar. Visit <yoursite>/admin/settings.php?section=calendar and set the calendar type default to 'test' (it should be 'gregorian' by default'). Enable self registration. Create a user using the registration steps and ensure that the calendar type set for their profile is 'test'. Create a user using the web service functionality and ensure the calendar type in their profile is 'test'. Log in using the DB authentication plugin and ensure the user is created with the 'test' calendar type.