FEATURE BlogCategoryCloud
FEATURE BlogCategory::$limit_to_holder
FEATURE Category listing page
Default sort for BlogCategory
API Apply extension to BlogTree instead of its child BlogHolder so they share functionality

Since its a has_many to BlogCategory, we can't relate to BOTH tree and holder,
but most of the getters make sense on BlogTree as well (e.g. showing categories if
they're not limited to a specific holder via configuration)
Allow limiting of "all tags" view (for SEO reasons)