This commit adds tests for managemails.models to improve the test
coverage of that model. There are some changes to the classes in
managemails.models too:
- add a method create_mailbox to MailboxManager
- properly handle uncommited mailaddresses in MailAddress.set_mailbox
and MailAddress.set_forward_addresses
This commit adds tests for managemails.forms. A refactoring TODO is
added to MailAddressFieldMixin and pragma: no cover has been added to
code paths not reachable if no new constants are added to the
MAILBOX_OR_FORWARDS constant array.