GDPR coming in effect on May 25th 2018

moodle.net can not store emails (including security and communications mailing lists) unless site administrator agrees to the privacy policy

Tasks:
1. During site registration ask to agree to privacy policy
2. During site upgrade (if registered in the past), ask to agree to privacy policy if not yet agreed
3. Send a confirmation email when admin agreed to the privacy policy (on registration or when updating registration)
4. Send a welcome email when email is added to security list (comms list already sends an email)
5. After all of the above is implemented on moodle.net and deployed to production:

• send one-time email to admins of all registered sites (and security/comms emails if different) and notify about the changes
• remove everybody who did not agree to privacy policy from mailing lists, also delete emails in the moodle.net database

(jleyva, please edit description of this issue or link to another issue about airnotifier changes)