New site setting: `limit_suggested_to_category` - ensures suggested

topics belong to the same category as the current topic and doesn't mix
in other categories.

config/locales/server.en.yml

@@ -765,6 +765,7 @@ en:
     max_private_messages_per_day: "The maximum amount of private messages users can create per day"
 
     suggested_topics: "Number of suggested topics shown at the bottom of a topic"
+    limit_suggested_to_category: "Only show topics from the current category in suggested topics"
 
     clean_up_uploads: "Remove orphaned uploads to prevent illegal hosting. WARNING: you might want to make a backup of your /uploads directory before enabled this setting."
     clean_orphan_uploads_grace_period_hours: "Grace period (in hours) before an orphan upload is removed."

config/site_settings.yml

@@ -34,6 +34,9 @@ basic:
   suggested_topics:
     client: true
     default: 5
+  limit_suggested_to_category:
+    client: false
+    default: false
   default_external_links_in_new_tab: false
   track_external_right_clicks:
     client: true

lib/suggested_topics_builder.rb

@@ -20,8 +20,13 @@ class SuggestedTopicsBuilder
     # Only add results if we don't have those topic ids already
     results = results.where('topics.id NOT IN (?)', @excluded_topic_ids)
                      .where(visible: true)
-                     .to_a
-                     .reject { |topic| @category_topic_ids.include?(topic.id) }
+
+    # If limit suggested to category is enabled, restrict to that category
+    if @category_id && SiteSetting.limit_suggested_to_category?
+      results = results.where(category_id: @category_id)
+    end
+
+    results = results.to_a.reject { |topic| @category_topic_ids.include?(topic.id) }
 
     unless results.empty?
       # Keep track of the ids we've added