Add command to confirm that Homu is listening

[edunham]

There are 2 cases in which users spam retry on a PR: When it really needs to be retried, and also when they're testing whether Homu is listening at all.

To reduce ambiguity, it would be nice to add a "@bors-servo: hello?" command, to which Homu would simply answer "@username: Hi there!" or equivalent. This way it would be clear which messages are simply debugging whether the bot is listening, vs which actually want a build to be kicked off.