Support triagebot assignment and shortcuts

It's convenient to be able to use triagebot shortcuts such as
`@rustbot author` and `@rustbot ready`, and to be able to use the
similar shortcuts for assignment, so let's enable those in this
repository.

Author: traviscross

triagebot.toml

@@ -8,6 +8,10 @@ allow-unauthenticated = [
 [rendered-link]
 trigger-files = ["text/"]
 
+[assign]
+
+[shortcut]
+
 [notify-zulip."T-cargo"]
 zulip_stream = 246057 # t-cargo
 topic = "RFC #{number} - {title}"