Paginate to get all of the PR comments

PR comments are returned 30 at a time. If a PR has a lot of comments then we
will generate duplicate comments, because we're not looking at all the comments.
This fix will fetch all the comments for us by paginating until the end.

* refactor the pagination to apply for user repos, org repos and comments
Yes. It would've been nice if we could use Octokit's auto pagination. Unfortunately [`pull_request_comments`](https://github.com/octokit/octokit.rb/blob/master/lib/octokit/client/pull_requests.rb#L179) method doesn't support auto pagination. However, the repo fetching calls do, so we could use auto pagination for those albeit in a separate PR.