While testing webhooks, I hit a few issues with the current signal

handlers, which this change fixes.
First, there's no guarantee that ReviewRequest.publish will have a user

passed, so we can't rely on the user when we get the signal callback.

This is also true when a review request is reopened. Now, we default to

the review request submitter.
We were providing a close_type field in the payload for the closed

handler of 'S' or 'D', which are internal states. We now pass

'submitted' or 'discarded' instead.
We also now use 'reply' instead of 'review' for reply payloads, to help

differentiate them. This leaves room for us to include a 'review' entry

as well, if we decide to.
Unit tests have been written for all the signal-based webhook

dispatchers to ensure they continue to work.

Unit tests pass.