Add a "Buildkite, test this please" like phrase that triggers a new CI run

Buildkite runs pull request presubmit checks only in response to a webhook, which means once after a new commit was pushed to a PR. There's no obvious way to trigger a CI run on e.g. older pull requests that existed before Buildkite was launched, or when for some reason the webhook didn't work.

@buchgr Can we add a trigger phrase like "Buildkite, test this please" (ideally with an "@"-mention to our bot) that - in addition to the normal webhook stuff - triggers a test run? Bonus points for automatically skipping the "verify" step when a trusted person writes it.

(@mattmoor FYI)