Disable redirect field unless the checkbox is set in setup

The logic for this box is meant to prevent the user from setting a redirect
unless they check the box first – but a simple forgotten line in set up to
actually disable the box if the checkbox is unticked made this not work in
the most-frequent case – when the page isn't a redirect, and the user is
trying to make it into one.

Bug: 61864
Change-Id: If250b24587a6f56fe545e4668d5e3517ffcff903
Merge "Disable redirect field unless the checkbox is set in setup"