Show INBOX subfolder under Shared/ namespace. Refs #2214 #1879

This is dovecot default setting for mail_shared_explicit_inbox.  TB17
seems to have problems with Shared/ subscriptions.
Don't set an error, if deleteACL() fails. Refs #2151

The DELETEACL can fail safely if the mailbox does not exist.
Fake empty group if MailDeliveryType is not "shared". Refs #2214

All subscriptions are canceled if MailDeliveryType changes.
createUserDefaultPseudonyms(): use username. Refs #2087

The default user pseudonym is now the username itself. First and Last
names are now ignored.