Geo: Impossible to delete orphaned registry through Admin

I've noticed this while developing https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9720.

Problem

When there are orphaned registries, the Geo Admin UI gives you a delete button to delete that registry:

But since you're on a Geo secondary looking at these records, you're getting this error:

You cannot perform write operations on a read-only instance

So the middleware is blocking your request. This happens both on Uploads and Projects.

Possible solution

We might want to make a family of API endpoints that allow write operations, e.g. at /geo_secondary/*. The middleware then needs to ignore those routes.