cgi escape server rendered text
rename subcribe to subcribing

This change is necessary because leastauthority will process stripe-generated
subscription information with a different resource than the resource used to submit
the subscription request. That resource was: "subscribing.py", processing of the
result will be conducted by subscription-complete.py.
Redirect the POST request from subcribing to subscription-complete.
render_POST is now handled by the subscription-complete resource.
All handlers inherit from HandlerBase for logging.