Add the ability for users to update their email preferences without logging in

[cjo2118]

We created a method on User called generate_email_authentication_token! that can be used to generate unique tokens for use with the /organization_subscriptions/:token url. When the user hits this url they will see a form that only allows them to change their email preferences.

We also made the tokens have an expiration time of 2 weeks, which can be changed arbitrarily.

In order to send out a mass email with these urls, a new token will need to be generated for each User. Then the tokens will need to be passed into the urls in each individual email.