Make destination page configurable

Problem/Motivation

We are using 'multiple_registration' module and we disabled the main registration page, '/user/register' and offer registrations only for one role with a dedicated registration page '/forum/register'. When users follow the affiliate link they are brought to '/user/register' which would show Access denied page.

We had to redirect '/user/register' to '/forum/register' to get going.

Proposed resolution

Make the destination url configurable in the UI so one can even choose to send users that follow the affiliate link to the homepage or some sales page if that was desirable.