Introduce safe host parsing command

[williammartin]

Description

See bug report: cli/cli#9248

TL;DR: The current host parsing function is non-idiomatic and hides errors opening the possibility for panics. We can't change the existing function without a breaking change so we will introduce a new function with signature func NewHost(hostname string) (*Host, error).

oauth/oauth.go

Lines 28 to 36 in adf5f73

	func GitHubHost(hostURL string) *Host {
	u, _ := url.Parse(hostURL)
	return &Host{
	DeviceCodeURL: fmt.Sprintf("%s://%s/login/device/code", u.Scheme, u.Host),
	AuthorizeURL: fmt.Sprintf("%s://%s/login/oauth/authorize", u.Scheme, u.Host),
	TokenURL: fmt.Sprintf("%s://%s/login/oauth/access_token", u.Scheme, u.Host),
	}
	}