Adds ReviewRating model

I can't just calling Rating because that would clash with the rating
field

bookwyrm/models/status.py

@@ -222,6 +222,22 @@ class Review(Status):
     pure_type = 'Article'
 
 
+class ReviewRating(Review):
+    ''' a subtype of review that only contains a rating '''
+    def save(self, *args, **kwargs):
+        if not self.rating:
+            raise ValueError('Rating object must include a numerical rating')
+        return super().save(*args, **kwargs)
+
+    @property
+    def pure_content(self):
+        #pylint: disable=bad-string-format-type
+        return 'Rated "%s": %d' % (self.book.title, self.rating)
+
+    activity_serializer = activitypub.Rating
+    pure_type = 'Note'
+
+
 class Boost(Status):
     ''' boost'ing a post '''
     boosted_status = fields.ForeignKey(