Web Hooks - honor Retry-After header when handling 429 Too Many Requests

Proposal

We currently consider all 4xx responses for web-hooks to be evidence they are misconfigured. Too many in a row result in the hook being disabled until users intervene to reenable them.

However, the response 429 - Too many requests should get different behavior:

we should:

• Back-off immediately (not after 3 such responses), since we have been told to back-off
• Honour the Retry-After header

Implementation details

We would add some special handling of the hook failure state when the log_data shows that the response code was 429 and there was a Retry-After header:

• Set the WebHook#disabled_until property to a time in future that honours Retry-After.
• Set WebHook#recent_failures to WebHooks::AutoDisabling::FAILURE_THRESHOLD so that the webhook is immediately backed off and will not retrigger until disabled_until.