chore: handle retry for GitHub issues

[mcdurdin]

As GitHub issues are loaded individually on webhook events now, we need a way to retry on error, because the same issue may not come up again for a long time as a webhook event, so if an error occurs we may never see the refreshed issue in question.

Once we do the same treatment for pull requests etc, then we'll have to deal with that there too. A generalized back-off retry pattern is probably best.