Completion system print_help_icon: should be changed to return string not print directly

The completion system uses a function completion_info::print_help_icon which prints the 'Your progress' help button.

A developer here is trying to implement a new course format and to use a proper renderer structure. In order to do this it would be helpful if the print_help_icon function were changed to display_help_icon (return html string rather than printing it directly).