Fix-316: paginated get with children

[mateusz-kleszcz]

Ticket

#316

Summary

This PR introduced pagination to the GET with children endpoint. It is based on another PR, but resolves some of its issues (lack of children nested more than one level, removes children when using /flat endpoint)

Test Plan

• create comments with nested children (threadOf property)
• try to fetch them with findAllInHierarchy endpoint, check if structure is unchanged
• add a pagination[pageSize] and pagination[page] query params, check if only paginated comments are returned