Add first name / last name support for mailboxes

[martbock]

If MUM is used in conjunction with services like Open-Xchange, those services may require a first name and a last name for their users. More concrete: Open-Xchange's console command createuser requires a --givenname and a --surname flag. Since MUM currently only supports one name field for mailboxes, you cannot use correct data here. Of course you could fill our name property in one field and fill the other with something like the email address, but that is quite ugly.

I think it makes sense to introduce the fields first_name and last_name on the Mailbox model.
These fields should also become available as integration placeholders.

Maybe there should also be a configuration option to switch between the current combined name field or one first_name field and one last name field.