PLAT-364 Add getCategoryByName back to WikiFactoryHub

This adds the getCategoryByName method back to WikiFactoryHub. There are some existing calls to this method that have not been removed so we need to continue to support this interface.

/cc @Wikia/platform-team @owend @inez 
I assume that @owend intends to change the default of `$new` to `true` (and the call sites) when he switches over.