Add settings endpoint so frontend can show/hide SSO login button. Ren…

[jessesuen]

• Add settings endpoint so frontend can show/hide SSO login button.
• Rename config to settings

This adds an endpoint /api/v1/settings, which returns the following:

{
  "url": "https://argocd.example.com",
  "dexConfig": {
    "connectors": [
      {
        "name": "Intuit GitHub",
        "type": "github"
      }
    ]
  }
}

[jessesuen]

See related issue #154 for how UI should use this endpoint once merged.