FEATURE: Show more context in Discourse topic oneboxes

Currently when generating a onebox for Discourse topics, some important
context is missing such as categories and tags.

This patch addresses this issue by introducing a new onebox engine
dedicated to display this information when available. Indeed to get this
new information, categories and tags are exposed in the topic metadata
as opengraph tags.

lib/onebox/engine/discourse_topic_onebox.rb

@@ -0,0 +1,59 @@
+# frozen_string_literal: true
+
+module Onebox
+  module Engine
+    class DiscourseTopicOnebox
+      include Engine
+      include StandardEmbed
+      include LayoutSupport
+
+      matches_regexp(%r{/t/.*(/\d+)?})
+
+      def data
+        @data ||= {
+          categories: categories,
+          link: link,
+          article_published_time: published_time.strftime("%-d %b %y"),
+          article_published_time_title: published_time.strftime("%I:%M%p - %d %B %Y"),
+          domain: html_entities.decode(raw[:site_name].truncate(80, separator: " ")),
+          description: html_entities.decode(raw[:description].truncate(250, separator: " ")),
+          title: html_entities.decode(raw[:title].truncate(80, separator: " ")),
+          image: image,
+          render_tags?: render_tags?,
+          render_category_block?: render_category_block?,
+        }.reverse_merge(raw)
+      end
+      alias verified_data data
+
+      private
+
+      def categories
+        Array
+          .wrap(raw[:article_sections])
+          .map
+          .with_index { |name, index| { name: name, color: raw[:article_section_colors][index] } }
+      end
+
+      def published_time
+        @published_time ||= Time.parse(raw[:published_time])
+      end
+
+      def html_entities
+        @html_entities ||= HTMLEntities.new
+      end
+
+      def image
+        image = Onebox::Helpers.get_absolute_image_url(raw[:image], @url)
+        Onebox::Helpers.normalize_url_for_output(html_entities.decode(image))
+      end
+
+      def render_tags?
+        raw[:article_tags].present?
+      end
+
+      def render_category_block?
+        render_tags? || categories.present?
+      end
+    end
+  end
+end