Ubuntu: rename to trojita-ubu

This is done in order to prevent filesystem clashes between the Ubuntu-branded
and the common desktop version.

BUG: 333073
REVIEW: 117418
Ubuntu: Enable viewing mailbox subdirectories

Mailbox view can now navigate through child mailboxes.

REVIEW: 117382
BUG: 332989

Fixed translation of Total & Unread message counts

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