Notifications for new issues and PRs

[dtolnay]

In serde-rs repos and my personal repos, usually when someone makes an issue or PR they don't ping my username. But I would still like to treat those as a notification because I am the one responsible.

It would be great to set via some config, whether local to the repo or global to the user or org, that says who wants to be notified on new issues/PRs.

[Mark-Simulacrum]

fef5538 made all serde-rs activity generate pings for @dtolnay. I suspect this'll be too much -- or at least that we'll want some sort of deduplication -- but I think it's a decent starting point.

One thing I came across recently that may be worth exploring as a way to expand triagebot's scope (relatively) easily is to replace our webhooks with GitHub's support for
PubSubHubbub/WebSub. It looks like that's no better than webhooks, though, since based on this StackOverflow post you need Admin access to the repositories you subscribe to.