Database column `mailbox`.`username` is not unique

[PhrozenByte]

mailbox.username currently isn't required to be unique in the database (uniqueness is currently only enforced by code), nor is the column even indexed. Querying the ViMbAdmin database using mailbox.username as (supposed to be unique) identifier is one of the main database queries any mail server software (Dovecot, Postfix, etc.) performs frequently (up to thousands of queries a day, depending on the number of users). mailbox.local_part should be indexed, too.