Add custom admin view for the "Group" model

- Ref: https://github.com/Microdisseny/django-groupadmin-users
- Adds ability to edit users within a particular group from the group admin page!

InvenTree/InvenTree/settings.py

@@ -138,6 +138,7 @@ INSTALLED_APPS = [
     'part.apps.PartConfig',
     'report.apps.ReportConfig',
     'stock.apps.StockConfig',
+    'users.apps.UsersConfig',
 
     # Third part add-ons
     'django_filters',               # Extended filter functionality

InvenTree/users/admin.py

@@ -1,3 +1,66 @@
 # -*- coding: utf-8 -*-
-# from __future__ import unicode_literals
-# from django.contrib import admin
+from __future__ import unicode_literals
+
+from django.utils.translation import ugettext_lazy as _
+
+from django.contrib import admin
+from django import forms
+from django.contrib.auth import get_user_model
+from django.contrib.admin.widgets import FilteredSelectMultiple
+from django.contrib.auth.models import Group
+
+User = get_user_model()
+
+
+class InvenTreeGroupAdminForm(forms.ModelForm):
+
+    class Meta:
+        model = Group
+        exclude = []
+        fields = [
+            'users',
+            'permissions',
+        ]
+
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+
+        if self.instance.pk:
+            # Populate the users field with the current Group users.
+            self.fields['users'].initial = self.instance.user_set.all()
+
+    # Add the users field.
+    users = forms.ModelMultipleChoiceField(
+        queryset=User.objects.all(),
+        required=False,
+        widget=FilteredSelectMultiple('users', False),
+        label=_('Users'),
+    )
+
+    def save_m2m(self):
+        # Add the users to the Group.
+        # Deprecated in Django 1.10: Direct assignment to a reverse foreign key
+        #                            or many-to-many relation
+
+        self.instance.user_set.set(self.cleaned_data['users'])
+
+    def save(self, *args, **kwargs):
+        # Default save
+        instance = super().save()
+        # Save many-to-many data
+        self.save_m2m()
+        return instance
+
+
+class RoleGroupAdmin(admin.ModelAdmin):
+    """
+    Custom admin interface for the Group model
+    """
+
+    form = InvenTreeGroupAdminForm
+
+    filter_horizontal = ['permissions']
+
+
+admin.site.unregister(Group)
+admin.site.register(Group, RoleGroupAdmin)