Use "reactions" for confirmation

[bertptrs]

Hoff comments on your PRs a lot when it's working. For example:

Not all of these have to be individual comments. Hoff could simply have leave one comment and keep updating it as it goes. However, that's hard and not what I'm suggesting.

The first message is basically useless beyond "yes I heard you." Other bots, like @dependabot or @bors-ng, leave a simple ":+1:" instead.

In addition to not being as visually spamming, it also doesn't cause notifications to be sent. All the while it still conveys the same information. I think Hoff should also do it this way.