Make "Start/Add Merge Train" button red when the latest pipeline in the merge request has not passed
Problem
When the latest pipeline in the merge request has not passed (e.g. failed by a broken code), it's not safe to add the merge request to the merge train because it's highly likely to fail the pipeline for merge train and thus the merge request will be rejected from the train and the following merge request's pipelines for merge train will be re-generated.
We should show warning sign or red background in the "Start/Add Merge Train" button to indicate users that it's not safe operation and better to see the green pipeline in the merge request at first.
Alternatively, we can hide the "Start/Add Merge Train" button when the latest pipeline has not passed.
UX Proposal
"Start/Add Merge Train" button should be type danger (red background color) when the latest pipeline in the merge request has not passed.
We had initially discussed the option to make the add/start merge train
button red based on the status of the pipeline (passed/failed). See #9186 (comment 156969526)
/cc @rverissimo @ogolowinski @nfriend