Pull Request for Issue #904.

Summary of Changes

Restructured the issues and pull requests webhook receivers:

• When inserting new records, we will only do so for opened, closed, and reopened events; we have had funky issues with race conditions when processing other event types as the initial one inserting the data
• When updating existing records, there are now different paths based on the action
  • opened, closed, reopened, edited, and synchronize (for pull requests) continue to behave as before
  • labeled and unlabeled events will now only update the item's labels versus trying to update all data
  • Other event types will hit a default case and report as unsupported (this includes milestoned and unmilestoned, there is no logic for updating milestones implemented into our webhook code at all right now so there's no loss here)

The pull request event listener will also now ignore certain actions as these could be problematic; these should only run on full insert/update/edit events.

This also hides synchronize events from the issue timeline, the same as GitHub natively does.

This should be a little more defensive against changing webhook events. We now have to explicitly code in support for new events versus them automagically being processed, plus this will now limit what some activities do (instead of updating all of the data on every event we only process explicit data for each event type).

Testing Instructions

All webhook types continue to receive and process data as before.