Create general rails controller dashboard

Creating a new dashboard in dashboards/general/rails-controller.dashboard.jsonnet. I've opted to retain the existing dashboards for now instead of removing them and potentially causing some confusion to devs.

See #139 for more background info

See web's rails-controller dashboard:

See the matching general dashboard (set with identical filters). Snapshot at https://dashboards.gitlab.net/dashboard/snapshot/aS8snzm1KzHgo4TcjFjbr1cqKygY8E0q though the top level pickers are not enabled on snapshots. See playground for https://dashboards.gitlab.net/d/test-general-rails-controller/test-general-rails-controller?orgId=1.