Expose the Koding application settings in the API

## Why was this MR needed?

When saving the GitLab application secrets in Koding, and authorising your admin user to have access to the UI, we want to let Koding enable the integration, and populate the url in GitLab for the user.

## What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/22705

See merge request !6555