refactor(config): simplify UI by using HiddenTabs

I've simplified the approach to #1493 (closed) by using the HiddenTabs component. I think it could still be simplified further, as we're keeping state in separate variables where I suspect we don't need to, since all the forms continue to be available. Also, I don't think we need to reload/rebuild the forms upon saving the data. That causes focus to return to the 'general' tab and all panes to scroll back to the top, which could be disruptive.

There are small visual differences: no fade at top/bottom of the scrollable pane; and the scrollbar is now styled like the rest of Kotahi (though I liked the scrollbar styling in !1090 (closed)).

Also, there are some labels in this page that need translations provided.