The spinner on "Add to merge train" button spins indefinitely

Problem

Initially reported at https://gitlab.slack.com/archives/CBZGR90A2/p1568044874112000

As one of the gitlab-com/www-gitlab-com#5177 (closed) feedback, it seems that sometimes the spinner on "Add to merge train" button spins indefinitely.

We do not have a clue how/when this happens, also we do not know if this is frontend or backend bug yet.

Steps to reproduce

1. Create a new project and enable "Merge pipelines will try to validate the post-merge result prior to merging"

2. Add the following .gitlab-ci.yml file. Note that the job does not have only: [merge_requests]:

   my job:
     script:
       - echo "Hello, world"

3. Create a new merge request and wait until the MR's pipeline completes.

4. Click the "Start merge train" button

5. If the text changes to "Added to the merge train by ", click "Remove from merge train"

6. Repeat steps 4 and 5 until the "Start merge train" button spins indefinitely. I find it usually takes 3 or 4 replications to get to this state.

Proposal

TBD

Workaround

Uncheck squash and merge, per #24084 (comment 236696568)