Retry PR creation API after rate limiting

[ivomurrell]

We've been running into abuse limiting when creating many PRs for a workspace, presumably as a result of secondary rate limiting. We use the throttling plugin from octokit to handle standard rate limiting but it doesn't seem to catch limiting behaviour that GitHub considers more abusive. From GitHub's best practices documentation:

Requests that create content which triggers notifications, such as issues, comments and pull requests, may be further limited and will not include a Retry-After header in the response. Please create this content at a reasonable pace to avoid further limiting.

Now requests are retried at a minimum of 5 seconds after the original, which is probably what we want even if the documentation is (intentionally) vague.

Closes #149

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[ivomurrell]

Closed by #164