Fixed #14370 -- Allowed using a Select2 widget for ForeignKey and ManyToManyField in the admin.

Thanks Florian Apolloner and Tim Graham for review and
contributing to the patch.

tests/admin_widgets/models.py

@@ -27,6 +27,7 @@ class Band(models.Model):
 
 class Album(models.Model):
     band = models.ForeignKey(Band, models.CASCADE)
+    featuring = models.ManyToManyField(Band, related_name='featured')
     name = models.CharField(max_length=100)
     cover_art = models.FileField(upload_to='albums')
     backside_art = MyFileField(upload_to='albums_back', null=True)