Add some API endpoints for Geo

To make the new Admin page in Vue no longer use the Rails controller, we need to expand the Geo Node API with:

• POST api/geo_nodes/:id/repair: To repair the authentication
• POST api/geo_nodes/id: To edit the Geo node

The links to these endpoints should included in the API response for quick access on the frontend (requested in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3902#note_53287807).

Also, it would be nice if the Geo node status would support Etag polling (as suggested in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3902#note_53295573).