On a config forms page, if the selected page doesn't exist (for example, if it

decides it should hide itself), the view would end up showing the nav and

nothing else. This is kind of a crummy experience, especially in the case where

the user was on one page but then it decided to hide itself.
This change adds a check to make sure we're trying to navigate to a visible

page, and if not, will select the first one in the list.

Used this with a config page that would hide itself when requested. Instead of

saving and then showing an empty page, it saves and then shows the first page

in the nav.