admin: Added realm option to prevent users from changing their email.

A realm option to prevent users from changing their email address is added.
Fixes #3777.

zerver/migrations/0058_realm_email_changes_disabled.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.5 on 2017-02-27 14:34
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0057_realmauditlog'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='realm',
+            name='email_changes_disabled',
+            field=models.BooleanField(default=False),
+        ),
+    ]