FEATURE: ability to restrict some tags to a category while allowing all others

A new checkbox has been added to the Tags tab of the category settings modal
which is used when some tags and/or tag groups are restricted to the category,
and all other unrestricted tags should also be allowed.
Default is the same as the previous behaviour: only allow the specified set of
tags and tag groups in the category.

app/serializers/category_serializer.rb

@@ -18,6 +18,7 @@ class CategorySerializer < BasicCategorySerializer
              :custom_fields,
              :allowed_tags,
              :allowed_tag_groups,
+             :allow_global_tags,
              :topic_featured_link_allowed,
              :search_priority