SeriesService.countSeriesOf(): move to CollectionService

The puzzle `927-e88d3d70` from #927 has to be resolved:

https://github.com/php-coder/mystamps/blob/ba083d8bc73280b035cffaff68780ba61b42677c/src/main/java/ru/mystamps/web/service/SeriesService.java#L38-L38

The puzzle was created by Slava Semushin on 24-Jul-18.