Merge the Transkribus and file imports

• Remove ProcessMode.Transkribus
• Remove the CreateImportTranskribus API endpoint
• Remove the [Partial]UpdateUserTranskribus API endpoint
• Remove User.transkribus_email
• Remove transkribus_import_email from the user serializer
• Remove Process.collection_id
• Remove the transkribus feature flag
• Remove the transkribus settings from the backend config.yml
• Add a data migration to update all ProcessMode.Transkribus processes to ProcessMode.Files, or just remove them
• Remove the transkribus-client dependency from requirements.txt, .gitlab-ci.yml, Dockerfile, Dockerfile.binary
• Allow the application/zip MIME type on file imports when tasks#158 (closed) is done