Create PostLoginRedirect Hook for changing the redirect behavior

Allows for modifying returnTo url, url parameters, and the redirect
type.

New return to condition 'signup' if redirecting after signup
Treated same as 'success', though now explicitly marking as such

Part of making GettingStarted not dependent on CentralAuth

Bug: 65619
Change-Id: I219353d9825918e22d4772611a38d72c4e674e10
Merge "Create PostLoginRedirect Hook for changing the redirect behavior"