Rewrite `bin/check` in Go

Part of #173 (closed)

The bin/check script is written in Ruby and depends on a significant quantity of lib/gitlab_net.rb. It is not used in an automated fashion anywhere, but may be used manually for debugging. As-is, it blocks !338 (closed) .

This MR adds go/cmd/check, replacing the Ruby implementation, which we want to delete.

I think that in the future, we may wish to remove the the check command entirely, but another option would be to make it remotely accessible, perhaps requiring administrator access to run it.