settings: preserve image format for thumbnails (#28081)

This allows PNG alpha channel to be kept.

combo/settings.py

@@ -184,6 +184,8 @@ HAYSTACK_CONNECTIONS = {
     },
 }
 
+# from solr.thumbnail -- https://sorl-thumbnail.readthedocs.io/en/latest/reference/settings.html
+THUMBNAIL_PRESERVE_FORMAT = True
 
 COMBO_DEFAULT_PUBLIC_TEMPLATE = 'standard'
 COMBO_PUBLIC_TEMPLATES = {