Notification preferences need to be more targeted and narrower

Currently, the only notification preference OCG has is:

> Receive optional notifications
> Receive broader announcements such as new event announcements, event reminders, and custom messages from organizers. You will still receive account, invitation, registration, speaker, refund, waitlist, cancellation, and reschedule updates.

This means the user has two choices:

1. Receive notifications of every new meetup event everywhere in the world
2. Don't receive organizer messages of the events they've attended

This is problematic, and likely to make organizer messages useless for reaching attendees. A reasonable set of preferences would be:

* Receive organizer messages and other notifications for groups whose events I've attended? Y/N
* Receive notifications for all new events on OCG? Y/N

Then, for each group, users should be able to turn on or off notifications for individual groups as a preference. For example, if a user moved from Seattle to Los Angeles, they will want to disable notifications from the Seattle meetup they used to go to.

This kind of notification management is one of the few things that Meetup does fairly well, so consider copying their model.