Add interface to manage mailing list

[bentsherman]

We currently manage our mailing list with a spreadsheet, and we just add and remove emails on demand. However, we might be able to incorporate the mailing list into the private website.

One way to do it would be to include the emails of all users (drawn from the users table) and an additional table (such as mailing_list) of emails for people who don't have an account but still want to receive emails. Users would need an option to not receive emails, such as for alumni. However, we might want to require all Active users to be able to receive emails.

We also might be able to just use a service like MailChimp. I haven't looked into it so I don't know how it would work.