Collection: change collections to be an extension of items

Each collection now has an item and the item's UID is the collections
UID. This lets us manipulate collections just like items, and as part of
transactions. This is significant because it lets us change them as part
of transactions!

django_etebase/models.py

@@ -27,20 +27,15 @@ UidValidator = RegexValidator(regex=r'^[a-zA-Z0-9]*$', message='Not a valid UID'
 
 
 class Collection(models.Model):
-    uid = models.CharField(db_index=True, blank=False, null=False,
-                           max_length=43, validators=[UidValidator])
-    version = models.PositiveSmallIntegerField()
+    main_item = models.ForeignKey('CollectionItem', related_name='parent', null=True, on_delete=models.SET_NULL)
     owner = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
 
-    class Meta:
-        unique_together = ('uid', 'owner')
-
     def __str__(self):
         return self.uid
 
     @cached_property
-    def main_item(self):
-        return self.items.get(uid=None)
+    def uid(self):
+        return self.main_item.uid
 
     @property
     def content(self):