What is the problem?
We have a hidden site setting `show_category_definitions_in_topic_lists`
which is set to false by default. What this means is that category
definition topics are not shown in the topic list by default. Only the
category definition topic for the category being viewed will be shown.
However, we have a bug where we would show that a category has new
topics when a new child category along with its category definition
topic is created even though the topic list does not list the child
category's category definition topic.
What is the fix here?
This commit fixes the problem by shipping down an additional
`is_category_topic` attribute in `TopicTrackingStateItemSerializer` when
the `show_category_definitions_in_topic_lists` site setting has been set
to false. With the new attribute, we can then exclude counting child
categories' category definition topics when counting new and unread
counts for a category.
