[dovecot] Enable virtual mailboxes.

[dovecot] Enable virtual mailboxes.

Virtual mailboxes are mailboxes created for new users automatically
based on their credentials. Userdb lookups aren't necessary.
Directories for virtual mailboxes are owned by a single system
user (`vmail`). This commit enables virtual mailboxes in `dovecot`:
- Static userdb instead of SQL query.
- Username column as the primary key of the address book. It has to be
unique to prevent virtual mailboxes overlapping.