Bug 1033475 - Allow user to configure email preferences from their profile

Update User Profile edit screen to allow email preference
Update text on account creation screen to indicate how to remove consent to email
Don't send email address in feedback email if they have not consented
Hide user email from most admin screens if they have not consented to receive email
Add consent_to_email field to UserProfile
Change OneAndDoneUserManager.get_query_set() to from select_related to prefetch_related due to https://code.djangoproject.com/ticket/15040

Should secure Auth - User admin to superusers only so that not everyone can see all the email addresses
This is a good point, but I worry that it won't make it obvious to admins that it is not an actual email. 

Currently it is a manual system for emailing users, so if an admin wants to reply to feedback, for example, they need to take the user's email address from the system and paste it into an email that they compose manually. We want it to be very obvious to an admin, therefore, when email consent has not been granted, to avoid them sending emails to an invalid address (such as `no-email@oneandone.mozilla.org`).

I will discuss the issue with @rbillings, but I'm guessing we will keep with the text that does not even resemble an email.