Allow importing negative sell transaction

Buy transactions should be negative and sell transactions
are generally positive but can be negative if fees are
higher than proceeds. Let OFX, QIF, and CSV importer
and not statement reader decide about transaction amount signs.

BUG: 351874
REVIEW: 127983

Signed-off-by: Łukasz Wojniłowicz <lukasz.wojnilowicz@gmail.com>