Add status for "not attending this year"

[ForestMars]

Since it's important for us to be able to message all NYC Camp members who have accounts on the site, but have not registered for this year's conference, we really need to provide a way for people who have accounts to indicate they are not attending this year. That way, we can follow up with those who still need to respond, without bothering those people who cannot attend and have been diligent in updating their status.

This should be an addition to the conference registration form currently on /user, with an option for "I cannot attend this year" and ideally, a selector for reasons (suggest using a test field list of this with dropdown.)

Getting an accurate list of attendees is especially important this year, given the emphasis on security, and since we need to do these follow ups asap, this is a top priority.

Also, if you are closely following this issue, you will recognize that we actually have 2 "delta" lists: 1 is the list of all people who have a nyccamp.org account but have not registered yes for this year's camp, and there is also a 2nd delta list of all users who are in our master mailchimp list but don't have an account on nyccamp.org (ie they are active in drupal nyc but didn't attend in 2012 or 2013.) The 2nd delta list does not really affect this ticket and is noted here just for clarification.