models: Rename Realm.restricted_to_domain field.

This renames Realm.restricted_to_domain field to
emails_restricted_to_domains, for greater clarity as to what it does
just from seeing the setting name, without having to look it up.

Fixes part of #10042.

zerver/migrations/0178_rename_to_emails_restricted_to_domains.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.13 on 2018-07-27 21:47
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0177_user_message_add_and_index_is_private_flag'),
+    ]
+
+    operations = [
+        migrations.RenameField(
+            model_name='realm',
+            old_name='restricted_to_domain',
+            new_name='emails_restricted_to_domains',
+        ),
+    ]