Fix extensions to work with category sort changes
Three different extensions were copy-pasting all of
CategoryPage::closeShowCategory() just to change which CategoryViewer
descendant was used. I changed the base class to use a member variable
for the CategoryViewer class, so this reduces code duplication too. I
only tested this on CategoryTree; MetavidWiki is certainly still broken,
from the looks of it (assumes $this->from is a string), but
PeopleCategories might be okay. Hard to avoid breaking extensions with
this sort of schema/index change.