Allow configuration of Sentry error tracking

We added some config for Workhorse error tracking: https://gitlab.com/charts/gitlab/issues/249#note_70629304

However I believe we are missing configuration for Unicorn, the frontend, gitaly, and other services.

• With omnibus you can configure it for gitlab_rails: https://docs.gitlab.com/omnibus/settings/configuration.html#error-reporting-and-logging-with-sentry
• Here is the gitaly config: gitlab-org/omnibus-gitlab!1616 (diffs)
• etc.