Resolve "Use Select2 everywhere"

Closes #102 (closed)

TODO: Do not load any stuff from external sources.

biscuit/core/forms.py

 from django import forms
 from django.contrib.auth import get_user_model
 from django.utils.translation import ugettext_lazy as _
+from django_select2.forms import Select2MultipleWidget
 
 from .models import Person, Group, School, SchoolTerm
 
@@ -73,6 +74,12 @@ class EditGroupForm(forms.ModelForm):
         model = Group
         fields = ['name', 'short_name', 'members', 'owners', 'parent_groups']
 
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.fields['owners'].widget = Select2MultipleWidget
+        self.fields['members'].widget = Select2MultipleWidget
+        self.fields['parent_groups'].widget = Select2MultipleWidget
+
 
 class EditSchoolForm(forms.ModelForm):
     class Meta: