Assignment: Add warning about students falling into Default group if group submissions are enabled but not required

Test on one device of your choice. Using master site and teacher user. Navigate to  Mobile testing course -> Assignment sections. Enter "Assignment requiring groups" activity. Check it shows a warning: The setting 'Students submit in groups' is enabled and some users are either not a member of any group, or are a member of more than one group. Please be aware that these students will submit as members of the 'Default group'. Enter " MDL-64811 Warning about..." activity. Check it shows a warning: The setting 'Require group to make submission' is enabled and some users are either not a member of any group, or are a member of more than one group, so are unable to make submissions. Now enter with teacher user to 3.6 site. Navigate to  Mobile testing course -> Assignment sections. Enter "Assignment requiring groups" activity. Check it shows a warning: The setting 'Require group to make submission' is enabled and some users are either not a member of any group, or are a member of more than one group, so are unable to make submissions.