Use topic excerpts and category desriptions in meta description tags for crawlers

app/controllers/list_controller.rb

@@ -252,6 +252,7 @@ class ListController < ApplicationController
     end
 
     @category = Category.query_category(slug_or_id, parent_category_id)
+    @description_meta = @category.description
 
     guardian.ensure_can_see!(@category)
 

app/controllers/topics_controller.rb

@@ -373,6 +373,7 @@ class TopicsController < ApplicationController
 
     respond_to do |format|
       format.html do
+        @description_meta = @topic_view.topic.excerpt
         store_preloaded("topic_#{@topic_view.topic.id}", MultiJson.dump(topic_view_serializer))
       end
 

app/views/layouts/application.html.erb

@@ -4,7 +4,7 @@
     <meta charset="utf-8">
     <title><%= content_for?(:title) ? yield(:title) + ' - ' + SiteSetting.title : SiteSetting.title %></title>
     <meta content="width=device-width, initial-scale=1.0" name="viewport">
-    <meta content="<%= SiteSetting.site_description %>" name="description">
+    <meta content="<%= @description_meta || SiteSetting.site_description %>" name="description">
     <meta content="" name="author">
 
     <%- if SiteSetting.enable_escaped_fragments? %>

app/views/layouts/crawler.html.erb

@@ -4,7 +4,7 @@
     <meta charset="utf-8">
     <title><%= content_for?(:title) ? yield(:title) + ' - ' + SiteSetting.title : SiteSetting.title %></title>
     <meta content="width=device-width, initial-scale=1.0" name="viewport">
-    <meta content="<%= SiteSetting.site_description %>" name="description">
+    <meta content="<%= @description_meta || SiteSetting.site_description %>" name="description">
     <meta content="" name="author">
 
     <%= canonical_link_tag %>