Add an auto merge indicator when merge checks failed

What does this MR do and why?

Implements Add auto-merge footer indicator to Merge Detail... (#423109 - closed).

This MR adds an Auto merge enabled hint into the merge details of an MR with auto merge enabled and failed merge checks.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Run this in Rails console (gdk rails c): Feature.enable(:merge_when_checks_pass)

2. Configure your project to support auto merge, set Pipelines must succeed merge check here:

   

3. Create an MR with this gitlab-ci.yml config:

test:
  stage: build
  script:
    - sleep 30
    - exit 1

4. Open the MR, click on Set auto-merge button
5. If your pipeline is pending, cancel the pipeline
6. If your pipeline is running, wait for it to fail
7. The message in the merge details should appear: Auto merge enabled