UX: Replace heuristic solution for extracting root domain.

spec/serializers/topic_link_serializer_spec.rb

@@ -0,0 +1,13 @@
+require 'rails_helper'
+
+describe TopicLinkSerializer do
+
+  it "correctly serializes the topic link" do
+    post = Fabricate(:post, raw: 'https://meta.discourse.org/')
+    TopicLink.extract_from(post)
+    serialized = described_class.new(post.topic_links.first, root: false).as_json
+
+    expect(serialized[:domain]).to eq("meta.discourse.org")
+    expect(serialized[:root_domain]).to eq("discourse.org")
+  end
+end