debugserver: Add zoekt services to debug server

Created by: keegancsmith

We have a convenience page at http://sourcegraph.example/com/-/debug/ for site admins to view individual service's debug pages. This adds zoekt to the list.

See individual commits for more details.

Test plan: Visited http://localhost:3080/-/debug/ and clicked through to the zoekt services as well as others. See screenshots below

Part of #3294 (closed)