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.

spec/lib/onebox/engine/discourse_topic_onebox_spec.rb

@@ -0,0 +1,51 @@
+# frozen_string_literal: true
+
+RSpec.describe Onebox::Engine::DiscourseTopicOnebox do
+  subject(:onebox) { described_class.new(url) }
+
+  describe "#data" do
+    subject(:data) { onebox.data }
+
+    let(:url) do
+      "https://meta.discourse.org/t/congratulations-most-stars-in-2013-github-octoverse/12483"
+    end
+    let(:expected_data) do
+      {
+        article_published_time: "6 Feb 14",
+        article_published_time_title: "04:55AM - 06 February 2014",
+        article_tags: %w[how-to sso],
+        card: "summary",
+        categories: [{ name: "praise", color: "9EB83B" }],
+        data1: "1 mins 🕑",
+        data2: "9 ❤",
+        description:
+          "Congratulations Discourse for qualifying Repositories with the most stars on GitHub Octoverse.     And that too in just over an year, way to go! 💥",
+        domain: "Discourse Meta",
+        favicon:
+          "https://d11a6trkgmumsb.cloudfront.net/optimized/3X/b/3/b33be9538df3547fcf9d1a51a4637d77392ac6f9_2_32x32.png",
+        ignore_canonical: "true",
+        image:
+          "https://d11a6trkgmumsb.cloudfront.net/optimized/2X/d/d063b3b0807377d98695ee08042a9ba0a8c593bd_2_690x362.png",
+        label1: "Reading time",
+        label2: "Likes",
+        link:
+          "https://meta.discourse.org/t/congratulations-most-stars-in-2013-github-octoverse/12483",
+        published_time: "2014-02-06T04:55:19+00:00",
+        render_category_block?: true,
+        render_tags?: true,
+        site_name: "Discourse Meta",
+        title: "Congratulations, most stars in 2013 GitHub Octoverse!",
+        url:
+          "https://meta.discourse.org/t/congratulations-most-stars-in-2013-github-octoverse/12483",
+      }
+    end
+
+    before do
+      stub_request(:get, url).to_return(status: 200, body: onebox_response("discourse_topic"))
+    end
+
+    it "returns the expected data" do
+      expect(data).to include expected_data
+    end
+  end
+end