Can't set identityProvider property for GitLab authorization

Created by: VeselaHouba

• Sourcegraph version: 3.1.1
• Platform information: Docker version 18.06.0-ce, build 0ffa825

Steps to reproduce:

1. Set up authentication with GitLab Oauth (working, tested)
2. Create external service for GitLab
3. Try to set identityProvider

  "authorization": {
    "identityProvider": {
      "type": "oauth"
    }

Expected behavior:

Settings are saved

Actual behavior:

Following error appears

1 error occurred: * identityProvider: Additional property identityProvider is not allowed 

and service config is not saved

Whole config for external service

{
  "url": "https://gitlab.mysite.com",
  "token": "XXX"
  "authorization": {
    "identityProvider": {
      "type": "oauth"
    },
    "ttl": "1h"
  }
}

Note: when leaving only ttl property, config is saved.