Fix how teams are displayed in requested reviewers

[mislav]

1. The --json export now only renders the login field for User types and name and slug fields for Team types.
2. The pr view command now renders team reviewers in the format of ORG/SLUG instead of the team name. This is so that the same value can be used in the pr create -r flag.

## BEFORE
$ gh pr view 123 --json reviewRequests
{
  "reviewRequests": [
    {
      "__typename": "User",
      "login": "monalisa",
      "name": "",
      "slug": "",
      "Organization": {
        "login": ""
      }
    },
    {
      "__typename": "Team",
      "login": "",
      "name": "Some team",
      "slug": "some-team",
      "Organization": {
        "login": "myorg"
      }
    }
  ]
}

## AFTER
$ gh pr view 123 --json reviewRequests
{
  "reviewRequests": [
    {
      "__typename": "User",
      "login": "monalisa"
    },
    {
      "__typename": "Team",
      "name": "Some team",
      "slug": "myorg/some-team"
    }
  ]
}

Followup to #3701

[samcoe]

Nice catch!