Ping aliases for `@rustbot ping the_alias`

[Centril]

Instead of having to remember @rustbot ping icebreakers-cleanup-crew it would be great if we could add a memorable alias for the group so that I could e.g. write @rustbot ping cleanup. We would configure this by adding a list of aliases under a ping section.

[Mark-Simulacrum]

fff4611 implements the alias key for the ping configuration; rust-lang/rust can start using it as soon as the change to triagebot.toml lands in the master of that repo. Could you make that PR?

[Centril]

I think so yeah; could you add something to the wiki docs btw?