[OJS.de - OA-S III - ap 3: ranking by metric] implementation and tests
[OJS.de - OA-S III - ap 4: sorting by metric] implementation and tests
[OJS.de] Fix memory leak in hook registry
[OJS.de - OA-S III - ap 4: sorting by metric] implement sorting by last month's statistics
[OJS.de - OA-S III - ap 5: recommend by author] implementation and tests
[OJS.de - OA-S III - ap 6: recommend by similarity] migrate to native search - implementation and tests