Replace BoundingBox by a generic polygon

arkindex/documents/api.py

arkindex/documents/serializers.py

arkindex/documents/surface_link.py

arkindex/documents/tests/test_surface_linker.py

arkindex/documents/tests/test_transcription_create.py

arkindex/documents/views.py

arkindex/images/importer.py

arkindex/images/models.py

arkindex/images/serializers.py

arkindex/images/tests.py

arkindex/project/polygon.py

arkindex/project/tests.py

arkindex/project/tools.py