Previously, we always attempted to save the configuration form for the

currently selected search backend, even when we were disabling search.

This lead a 500 error and search not being able to be disabled (as we

were only checking the validity of the form when search was enabled).

Now we only save the sub-form configuration when search is enabled,

which lets search be disabled successfully.

I was able to disable search with this patch applied. Without it I

could not.