settings: Migrate to create_stream_policy structure.

This commit replaces the `create_stream_by_admins_only` setting with a
new `create_stream_policy` setting, which mirroring the structure of
the existing `invite_to_stream_policy`.

This is important preparation for migrating the waiting period feature
to be its own independent setting.

Fixes #12236.

zerver/migrations/0216_add_create_stream_policy.py

@@ -0,0 +1,19 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.20 on 2019-05-06 13:15
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0215_realm_avatar_changes_disabled'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='realm',
+            name='create_stream_policy',
+            field=models.PositiveSmallIntegerField(default=1),
+        ),
+    ]