Refactor validate_entry on user json_uploads

[luisa-beerboom]

In user/base_json_upload.py
The method is way too long and hard to understand.
It should hopefully be possible to make it shorter/more readable/more efficient.

Maybe consider doing the same for other json_uploads

[luisa-beerboom]

In accordance with this comment, we should probably wait for the new concept before doing the cleanup, because @MSoeb seems to imply that a complete overhaul of the user import code may be necessary