Add support for django-simple-captcha

We switched recently to Mailman3 with postorious and hyperkitty. At that point in time, the amount of spam increased significantly because bots were able to sign up and send messages over hyperkitty.

We also had the problem of abuse reports because bots were able to start the registration accounts to existing mail addresses which didn't belong to them. The confirmation was received by comcast and then there system automatically send out abuse reports and marked our mailing list server in various spam filter systems.

Our hope is that both can be avoided when the forms for signup, subscribe and forgot password have a captcha check before the confirmation mail is sent out.