MailmanProvisioner should lowercase email addresses for use with Mailman API

The Mailman 3 core API when it consumes email addresses sent to it by the MailmanProvisioner lowercases the email address. Later when the API returns the email address to MailmanProvisioner comparisons with email addresses linked to the CoPerson record can fail if the linked email addresses are mixed case.

The MailmanProvisioner should lowercase email addresses before sending them through the Mailman core API and when comparing with what is returned.