Suppress search in topic unless we have more than 10 posts

(configurable in site setting)

lib/search.rb

@@ -46,6 +46,10 @@ class Search
     @search_context = @opts[:search_context]
     @limit = Search.per_facet * Search.facets.size
     @results = GroupedSearchResults.new(@opts[:type_filter])
+
+    if Topic === @search_context && @search_context.posts_count < SiteSetting.min_posts_for_search_in_topic
+      @search_context = nil
+    end
   end
 
   # Query a term