Use CreateMetaDataBulk to create metadata in Transkribus imports

While working on arkindex/tasks#149 (closed) and arkindex/tasks#150 (closed), I noticed that importing transcripts was very slow compared to the rest of the script. The backend logs show that CreateMetaData seems to be taking most of the time, and we could use CreateMetaDataBulk instead to create all metadata for all pages, regions and lines at once.