WIP oaiharvester: workflows improvements

Work in progress for #2242 
You are completely right. Actually, I even just changed the name :-P 

We can leave it empty string in the model and let the application logic warn user. Besides, the interface will require this value to be filled and it should remain not nullable.

However, for the upgrade script, I will suggest to still hardcode a name to make old sites seamlessly work using the default workflow (which will be under `oaiharvester/workflows/`).