🔧 chore(migrations): add migration file 0014_archive_event_institution_relationendpoint_and_more.py

🔧 chore(models.py): rename HasPreprint model to RelationEndpoint for better semantics
🔧 chore(models.py): update Relation model to use RelationEndpoint model for has_preprint and is_preprint_of fields
🔧 chore(sqlite.py): update create_relation_objects function to use RelationEndpoint model instead of HasPreprint model for has-preprint and is-preprint-of relations