FEATURE: Adds an API to exclude a tag from a TopicQuery

To exclude a tag from a topic list, add the `exclude_tag` query
parameter. For example: `latest?exclude_tag=music`

lib/topic_query.rb

@@ -60,7 +60,8 @@ class TopicQuery
          tags
          match_all_tags
          no_subcategories
-         no_tags)
+         no_tags
+         exclude_tag)
   end
 
   def self.valid_options
@@ -689,6 +690,10 @@ class TopicQuery
         # the following will do: ("topics"."id" NOT IN (SELECT DISTINCT "topic_tags"."topic_id" FROM "topic_tags"))
         result = result.where.not(id: TopicTag.distinct.pluck(:topic_id))
       end
+
+      if @options[:exclude_tag] && tag = Tag.find_by(name: @options[:exclude_tag])
+        result = result.where.not(id: TopicTag.distinct.where(tag_id: tag.id).pluck(:topic_id))
+      end
     end
 
     result = apply_ordering(result, options)