Use a unique request to load elements children for tree & navigation

Currently 2 nearly identical requests are made in parallel to load the children of an element while browsing:

• /api/v1/elements/XXX/children/?page=1&with_children_count=true&with_transcription_sources_count=true
• /api/v1/elements/XXX/children/?page=1&with_best_classes=true

We should have a unique request here