MR widget merge status icon should be red failure when running/pending with external pipeline

The MR widget merge status icon should be red failure when running/pending with external pipelines. The reason they are green now is because we usually show a "Merge when pipeline succeeds"(MWPS) button with GitLab pipelines.

Also investigate not_found on whether it should be red/green (probably red).

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/36487 where we will possibly change this over to warning icons.