API integration with Mailchimp

[asgrim]

Add a button create the mailshot for Mailchimp. Depends on #105

Need to allow admin to specify the date to schedule the email, and potentially add additional sections (e.g. for PHPSC information, special meetup info etc.).

Otherwise, we should use a standard template (possibly look at redesigning this at the same time).

[asgrim]

Or drop mailchimp and use transaction email to do clever reminders

[asgrim]

Suggestion from @MrNorm - click a link in the email that confirms attendance, easy simple