- move active flag into managemails.models.ActivateAbleMixin
- refactor Mailbox and Mailaddress to use ActivateAbleMixin
- implement managemails.models.Mailbox.__str__
- add managemails.admin.ActivationChangeMixin to provide activate and
deactivate actions for model admin classes
- add activate and deactivate actions for managemails.admin.MailboxAdmin
- add managemails.admin.MailAddressAdmin and inlines for MailAddressMailbox and
MailAddressForward management on the MailAddress management admin page