Also use a reaction for confirmation when the PR ends up in the merge queue

[isomorpheme]

In #272 I decided to send a reaction only if the PR in question is the first to be merged, so that we do still provide the position in the merge queue otherwise. But it's annoying that this still leads to an unnecessary ping:

It would be better if Hoff just reacts with 👍 in this case as well, and in addition sends a message just stating the position in the queue ("Waiting for rebase behind N pull requests...").

The check for the queue position happens here:

hoff/src/Logic.hs

Lines 1189 to 1195 in 43b6787

	case (queuePosition, source) of
	(0, Just reactable) -> ReactionFeedback reactable GithubApi.PlusOne
	_ ->
	CommentFeedback $
	format
	"Pull request approved for {}{} by @{}{}, {}."
	[approvalCommand, priorityMsg, approvedBy, retriedByMsg, queuePositionMsg]

The Feedback type would probably have to be modified to allow a reaction and a comment, or something else in the logic loop would have to change so that we also go back and send a comment in a separate step.

[isomorpheme]

in addition sends a message just stating the position in the queue ("Waiting for rebase behind N pull requests...").

This might not even be necessary, since (I think) you'll get a speculative rebase in that case, which already results in a message indicating position:

hoff/src/Logic.hs

Line 1208 in 5fcf08e

"Speculatively rebased as {} behind {} other {}, waiting for CI …"