tracking-issue: Track Pull requests

Created by: tsenart

This changeset adds pull request support to the tracking-issue tool. It does a lot of refactorings that benefited the cause, and changes the library we use for GraphQL requests since the other one didn't support this use case.

In addition to printing pull requests in the top level of each assignee's workload, it also nests linked pull requests under issues as a sub-list.

This was requested by the @sourcegraph/distribution team, specifically @slimsag.