fix: Fix Source matching query does not exist error

[ziafazal]

While setting up course discovery for first time refresh_course_metadata command breaks with this error.

Traceback (most recent call last):
  File "/openedx/discovery/course_discovery/apps/course_metadata/management/commands/refresh_course_metadata.py", line 37, in execute_loader
    run_loader()
  File "/openedx/venv/lib/python3.8/site-packages/backoff/_sync.py", line 105, in retry
    ret = target(*args, **kwargs)
  File "/openedx/discovery/course_discovery/apps/course_metadata/management/commands/refresh_course_metadata.py", line 34, in run_loader
    return loader_class(*loader_args).ingest()
  File "/openedx/discovery/course_discovery/apps/course_metadata/data_loaders/api.py", line 51, in __init__
    self.default_product_source = Source.objects.get(slug=settings.DEFAULT_PRODUCT_SOURCE_SLUG)
  File "/openedx/venv/lib/python3.8/site-packages/django/db/models/manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/openedx/venv/lib/python3.8/site-packages/django/db/models/query.py", line 435, in get
    raise self.model.DoesNotExist(
course_discovery.apps.course_metadata.models.Source.DoesNotExist: Source matching query does not exist.

Reason for this is we don't have and entry with settings.DEFAULT_PRODUCT_SOURCE_SLUG in Source of coursemetadata app.

This PR creates entry with settings.DEFAULT_PRODUCT_SOURCE_SLUG in Source if it does not exist to get rid of Source matching query does not exist error.

[DawoudSheraz]

name is not necessarily the same as slug (edX vs edx).

[dyudyunov]

@ziafazal are you going to create a backport-PR to Quince?

[DawoudSheraz]

please set name to 'edX' and slug to 'edx' to ensure it does not cause issue when attempting to create the first object.