Add color field to Element Types

arkindex/documents/migrations/0007_elementtype_color.py

arkindex/documents/models.py

arkindex/documents/serializers/light.py

arkindex/documents/tests/test_corpus.py

arkindex/documents/tests/test_element_type.py

arkindex/process/tests/test_transkribus_import.py

arkindex/project/default_corpus.py

arkindex/sql_validation/process_elements_filter_type.sql

arkindex/users/tests/test_registration.py