Eliminate N+1 queries with authors and push_data_payload in Events API

Stan Hurequested to merge
sh-fix-events-nplus-one into master

As seen in http://profiler.gitlap.com/20180531/652141fa-a197-47bc-9061-8e9fd681a49f.txt.gz, we have significant N+1 queries in the Events API.

Builds on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19345

Edited by Stan Hu

Merge request reports