Fix feedback on MRs of nightly slots
The logic so far is to find the latest comment of @lhcbsoft and edit it in place by extending with the new message. This is confusing when ci-test is run, since then lhcbsoft replies to the trigger comment and susbsequent nightly "validation started" messages all go there, even if it's a resolved discussion.
This MR improves this by only ever extending plain comments. Whenever such a comment is replied to, it's converted to a discussion and subsequent "validation started" messages go in a new comment below that discussion. Also, ci-test lhcb-master-mr validation started messages will bunch up together with the rest of the messages from oridnary slots.