[RelDB] Remove back relations from code

At some point we should eliminate the back relations from the backend code.
Some tasks that are part of that:

• Form everything the checker.py Checker class does into database constraints and the like (perhaps, in some cases move the checks into the field validate method), then delete the checker.
• export_helper.py: replace get_fields_for_export with Model.get_writable_fields() in export_meeting and its helper methods, then remove get_fields_for_export. For now the checker stops this from happening.

Start by reading checker.py and the Checker class to understand which checks might become database constraints or field validation. Then inspect export_helper.py, especially get_fields_for_export, export_meeting, and its helper methods, alongside Model.get_writable_fields(). Done means the checker and obsolete export helper are removed after both task areas are replaced.