Config errors in external services no longer appear to show up in admin UI

Created by: beyang

Configuration errors in external services no longer appear in admin UI.

Repro:

• Add an external service with the following config:

{
  "authorization": {
    "ttl": "1s",
    "identityProvider": {
      "type": "oauth"
    }
  },
  "token": "$TOKEN",
  "url": "https://gitlab2.sgdev.org"
}

• View site admin UI (http://localhost:3080/site-admin). No error shows up, but logs show "Repository authz config was invalid (errors are visible in the UI as an admin user, you should fix ASAP). Restricting access to repositories by default for now to be safe.". If you add a log.Printf("%+v", seriousProblems) to this line, you can see the actual error (the GitLab URL doesn't exist in the auth.providers list in the critical site config).

This is a high-priority bug fix, because without the error, the code host configuration will fail silently, leading to unexpected behavior.