Add group role assignments overview and managing views

aleksis/apps/alsijil/forms.py

aleksis/apps/alsijil/models.py

aleksis/apps/alsijil/preferences.py

aleksis/apps/alsijil/rules.py

...apps/alsijil/templates/alsijil/class_register/groups.html

...sis/apps/alsijil/templates/alsijil/group_role/assign.html

...s/alsijil/templates/alsijil/group_role/assigned_list.html

...ijil/templates/alsijil/group_role/assignment_options.html

aleksis/apps/alsijil/templates/alsijil/group_role/chip.html

...alsijil/templates/alsijil/group_role/edit_assignment.html

aleksis/apps/alsijil/urls.py

aleksis/apps/alsijil/util/predicates.py

aleksis/apps/alsijil/views.py