VID-1906 Add hook to separate parser cache on new gallery pages

Helper class is not the correct place for disabling/modifying parser cache because cache is already checked once it gets there. 
Also this change will cache new gallery pages with a modified cache key to avoid conflict with default pages, while still reaping the benefits of parser caching.
All the other hooks for this extension are found in the WikiaPhotoGalleryHelper class. What about moving this function there to be consistent? Also, the naming of this function doesn't follow the usual convention of strictly camelCase for functions.