support notify_on_conflict without removing auto merge label

[chdsbd]

When merge.notify_on_conflict is enabled, Kodiak will remove the auto merge label and leave a comment when there is a merge conflict.

We remove the auto merge label to prevent Kodiak from evaluating the pull request and leaving a comment for each evaluation. This prevents Kodiak from adding comments infinitely.

The request for this issue is to find a way to leave a comment without removing the auto merge label. We don't want to store state for every pull request, because that's an unbounded amount of data to store. Can we store state in a pull request comment?