Profile Setting Option to receive email on material Privacy and Terms changes

[frankwiles]

Add an option to the user's profile/settings page where they can opt into being notified by email about material (i.e. not just a typo) change to the privacy policy or terms of use.

This allows human discretion on whether or not to notification is necessary (i.e. if the mailing address changes we don't likely need to spam everyone).

This also does not complicate the registration process or interrupt site use for the majority of users who likely are not concerned about changes to the policies.