Ignore comments left by deleted GitHub users
Ignore comments left by deleted GitHub users

This patch addresses a corner-case issue related to PR review comments that were left by users who deleted their GitHub accounts. The GitHub UI attributes these comments to https://github.com/ghost, but the REST API returns `null` for the `user` field in review comment API responses.

For now, we just ignore these comments when computing the commenters to display on the UI.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #68 from JoshRosen/handle-deleted-github-users and squashes the following commits:

16502d8 [Josh Rosen] Handle comments left by deleted GitHub users.

databricks/spark-pr-dashboard,databricks/spark-pr-dashboard,databricks/spark-pr-dashboard