N+1 SQL queries in ContentController#widget.json

This may be a regression from !23762 (merged). I think the issue is that MergeRequestSerializer doesn't preload the related pipelines/builds here: https://gitlab.com/gitlab-org/gitlab/blob/b99011af62935de0b15e8a314ffb7df1f8a3f303/app/controllers/projects/merge_requests/content_controller.rb#L36

Attached is the performance bar data from this query. There is 150+ queries just for ci_job_artifacts:

$ cat /tmp/merge_requests_22465_perf_bar_1581383074704.json  | jq . | grep sql | sort | uniq -c | sort -nr | more | grep job_artifacts | wc -l
     170

merge_requests_22465_perf_bar_1581383074704.json

/cc: @iamricecake