Open the thumbnail PUT URL to element admins

arkindex/documents/api/elements.py

arkindex/documents/api/entities.py

arkindex/documents/serializers/elements.py

arkindex/documents/tests/test_create_elements.py

arkindex/documents/tests/test_entities_api.py

arkindex/documents/tests/test_retrieve_elements.py

arkindex/images/api.py

arkindex/process/serializers/imports.py

arkindex/process/tests/test_create_process.py