MergeRequestsController: {head,base}_pipeline_downloadable_path_for_report_type is slow

From gitlab-com/gl-infra/production#895 (comment 183674697), rendering the security/codeclimate/etc. reports takes about 17% of https://gitlab.com/gitlab-org/gitlab-development-kit/merge_requests/704.json?serializer=widget:

I think removing the legacy reports via https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14291 might help significantly here since there are a number of ActiveRecord loads that would go away, but perhaps we need to do more caching here.

/cc: @ayufan