Improve support for groupformation of only a sub-set of students of a course

[jkonert]

If a course contains several group formations, teachers usually restrict accessability to certain Moodle groups, e.g. "exercise tuesday". Then the settings of the groupformation should respect this setting and give correct visual feedback.
1.) number of potential students should be displayed corresponding to the visibility/accessability filters (not "111 students in course" but "26 students can access this activity due to your set restrictions"
2.) if the filter-box in settings about "only consider students for grouping that answered at least one question" is not checked, then make sure that only the students that can see/access the acitivity are grouped randomly (see 1.)). Otherwise the random groups would contain all other students, even the ones from other exercise times what results in multiple groupings for them and the groups are mixed among times and thus are useless. Currently teachers need to additionally set the group mode to "seperate groups" and the grouping (Gruppierung) to certain set of groups (whose members are then considered as the cohort).