Improve observability of external contributions beyond the #good-first-issue-feed Slack channel.

Created by: valerybugakov

Context

Apart from the #good-first-issue-feed Slack channel, there're no tools to monitor activity on good-first issues and pull requests by external contributors. It leads to delays in reviews/responses from the Sourcegraph team and sometimes to abandoned pull requests.

Success criteria

1. We have a clear way to track pull requests made by external contributors.
2. We have a documented process that ensures responsiveness on good first issues.