Adding new groups for managing permissions

Created the 'access_managers' and 'superevent_managers' groups. These
groups will be given appropriate permissions for managing external
access to gracedb and creating/updating superevents, respectively.

gracedb/migrations/auth/0016_create_access_and_superevent_groups.py

+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.14 on 2018-08-02 18:33
+# Create new groups for managing external access to superevents
+# and for allowing non-Test superevent creation and update. We
+# also add users to those groups
+
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+GROUPS = {
+    'access_managers': [],
+    'superevent_managers': ['emfollow'],
+}
+
+def add_groups(apps, schema_editor):
+    Group = apps.get_model('auth', 'Group')
+    User = apps.get_model('auth', 'User')
+
+    for group_name, usernames in GROUPS.iteritems():
+        g, _ = Group.objects.get_or_create(name=group_name)
+        users = User.objects.filter(username__in=usernames)
+        g.user_set.add(*users)
+
+
+def remove_groups(apps, schema_editor):
+    Group = apps.get_model('auth', 'Group')
+    User = apps.get_model('auth', 'User')
+
+    for group_name in GROUPS:
+        g = Group.objects.get(name=group_name)
+        g.delete()
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('auth', '0015_update_emfollow_accounts'),
+    ]
+
+    operations = [
+        migrations.RunPython(add_groups, remove_groups),
+    ]