[many] Simplify backup system by using object's natural order (c.f. #433)

beat/web/common/models.py

@@ -381,7 +381,9 @@ class Versionable(Shareable):
 
     class Meta(Shareable.Meta):
         abstract = True
-        ordering = ['name', '-version']
+
+        # setup ordering so that the dump order respects self dependencies
+        ordering = ['previous_version_id', 'fork_of_id', 'id']
 
 
     #_____ Static Methods __________
@@ -552,7 +554,6 @@ class Contribution(Versionable):
 
     class Meta(Versionable.Meta):
         abstract = True
-        ordering = ['author__username', 'name', 'version']
         unique_together = ('author', 'name', 'version')