Fix ALTO xml preview

[silviot]

@chiruzzimarco The ALTO preview is not currently working. It errors with:

Traceback (most recent call last):
  File "manage.py", line 10, in main
    from django.core.management import execute_from_command_line
ModuleNotFoundError: No module named 'django'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "manage.py", line 21, in <module>
    main()
  File "manage.py", line 12, in main
    raise ImportError(
ImportError: Couldn't import Django. Are you sure it's installed and available on your PYTHONPATH environment variable? Did you forget to activate a virtual environment?

If you run it again you get a different problem:

[INFO] purging batch 'batch_lamasca_0001_ver01'
[ERROR] purge failed: Batch matching query does not exist.
[ERROR] Batch matching query does not exist.
Traceback (most recent call last):
  File "/opt/openoni/core/batch_loader.py", line 490, in purge_batch
    batch = self._get_batch(batch_name)
  File "/opt/openoni/core/batch_loader.py", line 207, in _get_batch
    batch = Batch.objects.get(name=batch_name)
  File "/opt/openoni/ENV/lib/python3.8/site-packages/django/db/models/manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/opt/openoni/ENV/lib/python3.8/site-packages/django/db/models/query.py", line 435, in get
    raise self.model.DoesNotExist(
core.models.Batch.DoesNotExist: Batch matching query does not exist.
[ERROR] purge failed: Batch matching query does not exist.
Traceback (most recent call last):
  File "/opt/openoni/core/batch_loader.py", line 490, in purge_batch
    batch = self._get_batch(batch_name)
  File "/opt/openoni/core/batch_loader.py", line 207, in _get_batch
    batch = Batch.objects.get(name=batch_name)
  File "/opt/openoni/ENV/lib/python3.8/site-packages/django/db/models/manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/opt/openoni/ENV/lib/python3.8/site-packages/django/db/models/query.py", line 435, in get
    raise self.model.DoesNotExist(
core.models.Batch.DoesNotExist: Batch matching query does not exist.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/opt/openoni/core/management/commands/purge_batch.py", line 40, in handle
    loader.purge_batch(batch_name)
  File "/opt/openoni/core/batch_loader.py", line 507, in purge_batch
    raise BatchLoaderException(msg)
core.batch_loader.BatchLoaderException: purge failed: Batch matching query does not exist.
CommandError: unable to purge batch. check the purge_batch log for clues