user settings: Add setting to make login notifications optional.

This adds a feature in the "Notification" section of "Settings" tab,
which lets user enable or disable login emails notification.

Tweaked by tabbott to simplify the test.

Fixes: #5795, progress towards #5854.

static/js/settings_notifications.js

@@ -20,6 +20,7 @@ var pm_mention_notification_settings = [
 
 var other_notification_settings = [
     "enable_digest_emails",
+    "enable_login_emails",
     "realm_name_in_notifications",
     "message_content_in_email_notifications",
 ];