notifications: Enable online push notifications by default.

For new user onboarding, it's important for it to be easy to verify
that Zulip's mobile push notifications work without jumping through
hoops or potentially making mistakes.  For that reason, it makes sense
to toggle the notification defaults for new users to the more
aggressive mode (ignoring whether the user is currently actively
online); they can set the more subtle mode if they find that the
notifications are annoying.

zerver/migrations/0258_enable_online_push_notifications_default.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.26 on 2019-12-11 00:41
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0257_fix_has_link_attribute'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='userprofile',
+            name='enable_online_push_notifications',
+            field=models.BooleanField(default=True),
+        ),
+    ]