Rename events module to registry

This prevents shadowing the import of the events module with the
events registry instance.
Fix string formatting for NotRegistered exception
Fix test http server, change to echo back request body
Remove supplying args and kwargs in logging

This conflicts with the LogRecord args key and could contain sensitive
data depending on the application.
Return the response and response code from _send_request

Synchronous requests can iterate over the responses and codes for
each url that was bound.
Add WEBHOOK_USER_AGENT and WEBHOOK_VERSION settings

These enable supplying a custom user agent string and version for
outbound requests.
Add custom ModelAdmin for easier editing from list view
Update, fix and add more tests