FEATURE: Embed topics list on remote sites via Javascript API. (#8008)

This adds support for a `<d-topics-list>` tag you can embed in your site
that will be rendered as a list of discourse topics. Any attributes on
the tag will be passed as filters. For example:

`<d-topics-list discourse-url="URL" category="1234">` will filter to category 1234.

To use this feature, enable the `embed topics list` site setting. Then
on the site you want to embed, include the following javascript:

`<script
src="http://URL/javascripts/embed-topics.js"></script>`

Where `URL` is your discourse forum's URL.

Then include the `<d-topics-list discourse-url="URL">` tag in your HTML document and it will
be replaced with the list of topics.

spec/requests/embed_controller_spec.rb

@@ -70,6 +70,33 @@ describe EmbedController do
     end
   end
 
+  context "#topics" do
+    it "raises an error when not enabled" do
+      get '/embed/topics?embed_id=de-1234'
+      expect(response.status).to eq(400)
+    end
+
+    context "when enabled" do
+      before do
+        SiteSetting.embed_topics_list = true
+      end
+
+      it "raises an error with a weird id" do
+        get '/embed/topics?discourse_embed_id=../asdf/-1234', headers: headers
+        expect(response.status).to eq(400)
+      end
+
+      it "returns a list of topics" do
+        topic = Fabricate(:topic)
+        get '/embed/topics?discourse_embed_id=de-1234', headers: headers
+        expect(response.status).to eq(200)
+        expect(response.headers['X-Frame-Options']).to eq("ALLOWALL")
+        expect(response.body).to match("data-embed-id=\"de-1234\"")
+        expect(response.body).to match("data-topic-id=\"#{topic.id}\"")
+      end
+    end
+  end
+
   context "with a host" do
     let!(:embeddable_host) { Fabricate(:embeddable_host) }
     let(:headers) { { 'REFERER' => embed_url } }