Update manip endpoint to fix path problem

One of the goals is to make the generated content directory directly hostable
via a web browser for performance. This means that the application needs to be
able to generate a file tree identical to the URL structure so that clients can
fetch a single URL and either use the cached image or trigger a manipulation
depending on whether it already exists.

With a url structure of /key and /key/manip for the source and manips respectively,
it becomes impossible to create a file tree matching it since 'key' must be both
the source file and a directory containing the manipulated images.

This updates it to use 'original' as a special manip name so the URL structure of
/key/original and /key/manip can match the directory structure

openapi.yaml

@@ -67,7 +67,7 @@ paths:
         '500':
           description: Server is not operating correctly
           headers: *headers-default
-  /image/{image_name}:
+  /image/{image_name}/original:
     head:
       summary: Returns metadata about the image request
       operationId: ImageHead