Refactoring & cleanup in routes & route groups + bug fixes

[tomasherceg]

We had several issues in route groups when used in combination with URL and route redirections:

config.RouteTable.AddGroup(
    "CmsAdmin", urlPrefix, "embedded://Shared.Infrastructure.Cms.Admin", routes =>
{
    routes.AddRouteRedirection(
        "Index", 
        "", // this URL was not concatenated with the group prefix, 
        "CmsAdmin_Articles");
    routes.Add("Preview", 
        "articles/preview", 
        "some_valid_virtual_path", // this cannot be null and requires a valid embedded resource path even though it is not used 
        presenterFactory: _ => new PreviewPopupPresenter());
});