Continue increasing number of maintainers and reviewers and update handbook as necessary

This is a continued effort for 2019-Q2 in reference to #4063 (closed)

Some TODOs carried over from #4063 (closed), which were suggested by @smcgivern

1. Set a guideline for the number of maintainers per FE / BE engineer in the handbook (on https://about.gitlab.com/handbook/engineering/workflow/code-review/ maybe?).
2. Project that forward and set a guideline for the number of trainees we'll need to meet that target given future hiring.
3. Keep bin/count-maintainers, but add the current stats to the handbook somewhere. (The code review page, or https://about.gitlab.com/handbook/engineering/projects/, or somewhere else?)

/CC: @timzallmann @dhavens @bmarnane @clefelhocz1 @rnienaber @mendeni @jameslopez @smcgivern @donaldcook @darbyfrey @DouweM @sengelhard

Should more managers be pinged? The above managers were involved in the former issue.