Adjust GitHub rate limiting behavior.

[ehuss]

This introduces two changes around GitHub rate limiting:

• Retries are only performed with 403 or 429 responses. GitHub's docs state that rate-limits will only return those responses. Previously, the client would retry any non-successful request (like a 404), which was frequently causing several needless requests to GitHub.
• Disable retries on the server. Since a rate-limit retry could take several minutes, that would almost certainly mean it would get cancelled due to the 10 second webhook rate limit. I don't think the other endpoints really need it either (a retry can sleep for a very long time). It is still enabled for the prioritization CLI tool.