ABC-153: don't use http redirects with 4xx/5xx status codes (#8178)

* don't use http redirects with 4xx/5xx status codes

* minor html syntax fix

app/oauth.go

@@ -564,7 +564,7 @@ func generateOAuthStateTokenExtra(email, action, cookie string) string {
 
 func (a *App) GetAuthorizationCode(w http.ResponseWriter, r *http.Request, service string, props map[string]string, loginHint string) (string, *model.AppError) {
 	sso := a.Config().GetSSOService(service)
-	if sso != nil && !sso.Enable {
+	if sso == nil || !sso.Enable {
 		return "", model.NewAppError("GetAuthorizationCode", "api.user.get_authorization_code.unsupported.app_error", nil, "service="+service, http.StatusNotImplemented)
 	}