OpenAtlas

This is a follow-up ticket from #1567 Enhanced Import.
Since all prerequisites are met, the import of a place hierarchy can be implemented.
As example structure, Stefan Eichert provided this CSV: https://docs.google.com/spreadsheets/d/1IIPGKvSRNi6Qeir6070VS4u7uvHdlpYenEhRkCaGJcI/edit?usp=sharing

• Add new import button "Place hierarchy" is now at place, if a column openatlas_class and parent_id is available ✅
• Link existing references with ID including page number ✅
• OpenAtlas Class name to know which level the place is ✅
• Parent import ID ✅
• Import hierarchy ✅

• Manual ✅
• Massive manual testing ✅