Show "real" avatars for GitHub Enterprise

[hackjutsu]

There is an ongoing discussion on the issue of broken GitHub Enterprise avatars. https://github.com/desktop/desktop/issues/821

Currently, we use a placeholder image which can be customized by .leptonrc.

    "enterprise": {
        "enable": false,
        "host": "github_enterprise_host",
        "token": "token_with_gist_enabled",
        "avatarUrl": "optional_avatar_url"  <---- customize your avartar!
    }