Add sidekiq dashboard
We shipped a lot of great improvements to the dashboards in !18 (comment 118832611). Namely additional dashboards for Redis, PG, and Gitaly.
We also worked on sidekiq, but ran into issues with gitlab-monitor: https://gitlab.com/gitlab-org/gitlab-monitor/issues/24
Those fixes should be part of 11.6, and so we should add a sidekiq dashboard.