Memorize lesson set for a submission

[ice-phoenix]

Original report by Mikhail Glukhikh (Bitbucket: mglukhikh, GitHub: mglukhikh).

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We can consider changed top-level packages as lessons here. When submission is created, we memorize which lessons were changed. Then we show only this lessons to the reviewer and do not consider changes in other lessons, even after resubmissions.

Can solve problem with "90% students put new tasks to old submissions via resubmissions".