Introduce flake8 return

Based on !2281 (merged)

I've made one commit per rule. I'm not sure I agree with:

• RET502
• RET503.

Please tell me what you think, I can easily remove them.

arkindex/images/managers.py

@@ -47,7 +47,7 @@ class ImageServerManager(models.Manager):
                 f'The URL "{url}" does not match any existing image server. '
                 "Please ask an instance administrator to register the IIIF server for this image."
             )
-        elif servers_count > 1:
+        if servers_count > 1:
             raise self.model.MultipleObjectsReturned(
                 f'The URL "{url}" matched multiple existing image servers'
             )