Update Opinion model to support versions

[grossir]

From the parent issue

data model changes: there are many ways we could model the opinion "versions". I think that the simplest is adding a field that points to the "main" version

class Opinion(models.Model):
    cluster = models.ForeignKey(
        OpinionCluster,
        help_text="The cluster that the opinion is a part of",
        related_name="sub_opinions",
        on_delete=models.CASCADE,
    )
    ....
    main_version = models.ForeignKey(self, null=True)

This has the advantage of being simple enough, and :

• You can find the "main" version via a simple query Opinion.objects.filter(main_version__isnull=True)
• Given the "main" version you can find all it's versions Opinion.objects.filter(main_version=main_version_id)
  these queries should be covered by an index
• The Opinion model already has all the metadata fields for comparing versions: dates, the uploaded document copy, the original URL, the extracted text
  all the versions would be grouped under the same OpinionCluster