Add handler to receive completion events

* Submit a completion when receiving a completion event from an XBlock.
* Handle legacy progress events.
* Convert handler to use a dispatch dict instead of an if-else chain.
* Extract masquerade checking from individual handlers.
* Gate submit_completion on waffle switch
* 404 on handler views when trying to submit completion without waffle
switch enabled.

OC-3087

Disallow calling submit_completion when waffle flag is disabled.

Add tests that trying to publish completion errors.