Add user setting option to always send push notifications.

Add option in user's settings for getting mobile push notifications
even if a Zulip browser is online.  Default is False.

Fixes: #1596.

zerver/migrations/0034_userprofile_enable_online_push_notifications.py

@@ -0,0 +1,19 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import models, migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0033_migrate_domain_to_realmalias'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='userprofile',
+            name='enable_online_push_notifications',
+            field=models.BooleanField(default=False),
+        ),
+    ]