Make TrackerModel.get_by_name select subclasses

This fixes an issue where the frontend would call get_by_name
and then get something that it did not know how to deal with.

Credit to John Morrissey for finding this one-line bug and writing
paragraphs and paragraphs about it.

Closes: https://openhatch.org/bugs/issue819

mysite/customs/models.py

@@ -196,7 +196,7 @@ def get_by_name(cls, tracker_name):
         def _pipe_things(a, b):
             return a | b
         joined = reduce(_pipe_things, query_parts)
-        return cls.objects.get(joined)
+        return cls.objects.select_subclasses().get(joined)
 
 class TrackerQueryModel(models.Model):
     '''This model just exists to provide a way to grab a QuerySet