FIX: Handle encoding on onebox excerpt

Oneboxes were breaking with bad encoding using the out of the box
excerpt on the topic. Mimicked core behaviors instead.

app/controllers/knowledge_explorer/knowledge_explorer_controller.rb

@@ -27,6 +27,9 @@ module KnowledgeExplorer
           raise Discourse::NotFound
         end
 
+        @excerpt = @topic.posts[0].excerpt(500, { strip_links: true, text_entities: true }) if @topic.posts[0].present?
+        @excerpt = (@excerpt || "").gsub(/\n/, ' ').strip
+
         query["topic"] = get_topic(@topic, current_user)
       end
 

app/views/knowledge_explorer/knowledge_explorer/get_topic.html.erb

@@ -1,3 +1,3 @@
 <% content_for :head do %>
-  <%= raw crawlable_meta_data(title: @topic["title"], description: @topic["excerpt"]) if @topic %>
+  <%= raw crawlable_meta_data(title: @topic["title"], description: @excerpt) if @topic %>
 <% end %>