Merge the Transkribus and file imports

README.md

arkindex_tasks/base.py

arkindex_tasks/enums.py

arkindex_tasks/export_transkribus.py

arkindex_tasks/import_files/base.py

arkindex_tasks/import_files/pagexml.py

arkindex_tasks/import_files/transkribus.py

tests/import_files/test_base.py

tests/import_files/test_pagexml.py

tests/import_files/transkribus/__init__.py

tests/import_files/transkribus/test_element.py

tests/import_files/transkribus/test_folder.py

tests/import_files/transkribus/test_importer.py

tests/test_base.py

tests/test_export_transkribus.py