Add convenience methods for redirecting from a Razor Page back to itself

[DamianEdwards]

Right now, you have to return Redirect("/url/to/page"). We should have a first-class method for this so that you don't have to pass in the URL, e.g. return RedirectToSenf();

[rynowak]

@pranavkm - Let's make this a follow up to your current set of changes.

Add a RedirectToPage() and RedirectToPage(object values). These call RedirectToPage(string) and RedirectToPage(string, object) respectively with the current page's name (from route values).