Download transkribus export in the import_transkribus task

Instead of:

1. downloading ZIP from transkribus
2. save as artifact on S3
3. downloading ZIP from S3

Let's simply be bad boys and use Transkribus as our CDN: the import_transkribus task can directly donwload the export mentionned by the export_arkindex task (as a JSON artifact).

On a more serious note, This should still only trigger 1 download on Transkribus server most of the time