Make provider names constants

arkindex/documents/fixtures/data.json

arkindex/documents/management/commands/build_fixtures.py

arkindex/documents/tests/commands/test_load_export.py

arkindex/documents/tests/tasks/test_corpus_delete.py

arkindex/process/admin.py

...rocess/migrations/0060_remove_repository_provider_name.py

arkindex/process/models.py

arkindex/process/providers.py

arkindex/process/tests/test_workers.py

arkindex/users/admin.py

arkindex/users/api.py

.../migrations/0018_remove_oauthcredentials_provider_name.py

arkindex/users/models.py

arkindex/users/serializers.py

arkindex/users/tests/test_credentials.py