This is a similar fix to 32d4810e2b
Why this change?
Prior to this change, there is a bug in `TopicsController#bulk`
where it does not dismiss new unred posts in sub-subcategories when the
`category_id` and `include_subcategories=true` params are present. This
is because the controller did not account for sub-subcategories when
fetching the category ids of the new topics that should be dismissed.
This commit fixes the problem by relying on the `Category.subcategory_ids` class
method which accounts for sub-subcategories.
fff578f5fb