Adding Join page for group

[mmilkin]

Adding Join action for a group.

Now with notifications

[mmilkin]

Added the beginning of the notification (this is now done through meetup, if we want to collect emails we could send emails ourselves. For now this is synchronous, not sure if we want to complexity of celery or async.

[nedbat]

I didn't want to collect emails, it feels like a barrier to people participating, so notifying through Meetup is the right way to do it.

[mmilkin]

I agree, we could query meetup for emails using the api but that might be intrusive.

[nedbat]

Well, meetup won't give out emails, there's no API to do that.