Users list and monthly subscription management

[asgrim]

The ultimate goal for having people log in was to allow people to manage whether they receive marketing or not. GDPR regulation being introduced makes this a perfect opportunity to ensure receivers of the monthly list are explicitly opted in. Mailing list at the moment is managed by Mailchimp, which I (think) is sufficient for showing opt-in records (which were collected on Eventbrite registrations originally).

This means that actually, any new attendees haven't been auto-subscribed (probably a good thing, as the PHPH site does not mention any marketing!)

I'd suggest the following changes (would help to have someone review the ideas for complying with GDPR stuff):

• When registering, have an opt-in checkbox to monthly newsletter box - not checked by default, with clear text and understanding
• In user settings, allow enabling this or disabling this
• Interfacing to Mailchimp API to subscribe/unsubscribe mailing list members with this. Ideally, also copy opt-in records to Mailchimp
• I expect we'd have to do the reverse (if the user with the email exists), maybe a cron, or maybe the Mailchimp API has a webhook for subscribe/unsubscribe?
• Existing users should not be opted in, unless they already receive the emails on Mailchimp