org settings: Add setting to prevent users from adding bots.

Fixes: #7908.

zerver/migrations/0143_realm_bot_creation_policy.py

@@ -0,0 +1,41 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.6 on 2018-03-09 18:00
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+from django.db.backends.postgresql_psycopg2.schema import DatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+
+def set_initial_value_for_bot_creation_policy(apps: StateApps, schema_editor: DatabaseSchemaEditor) -> None:
+    Realm = apps.get_model("zerver", "Realm")
+    for realm in Realm.objects.all():
+        if realm.create_generic_bot_by_admins_only:
+            realm.bot_creation_policy = 2   # BOT_CREATION_LIMIT_GENERIC_BOTS
+        else:
+            realm.bot_creation_policy = 1   # BOT_CREATION_EVERYONE
+        realm.save(update_fields=["bot_creation_policy"])
+
+def reverse_code(apps: StateApps, schema_editor: DatabaseSchemaEditor) -> None:
+    Realm = apps.get_model("zerver", "Realm")
+    for realm in Realm.objects.all():
+        if realm.bot_creation_policy == 1:  # BOT_CREATION_EVERYONE
+            realm.create_generic_bot_by_admins_only = False
+        else:
+            realm.create_generic_bot_by_admins_only = True
+        realm.save(update_fields=["create_generic_bot_by_admins_only"])
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0142_userprofile_translate_emoticons'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='realm',
+            name='bot_creation_policy',
+            field=models.PositiveSmallIntegerField(default=1),
+        ),
+        migrations.RunPython(set_initial_value_for_bot_creation_policy,
+                             reverse_code=reverse_code),
+    ]