This commit adds support for excluding categories when using the
`category:` filter with the `-` prefix. For example,
`-category:category-slug` will exclude all topics that belong to the
category with slug "category-slug" and all of its sub-categories.
To only exclude a particular category and not all of its sub-categories,
the `-` prefix can be used with the `=` prefix. For example,
`-=category:category-slug` will only exclude topics that belong to the
category with slug "category-slug". Topics in the sub-categories of
"category-slug" will still be included.
963bb3406e