FEATURE: Small improvements to the topic list embed (#12881)

* FEATURE: Small improvements to the topic list embed

- Ability to wrap the list in a custom class so you can styles different
lists using specific CSS

- Adds a topic link to the thumbnail when using the complete template

* FIX: Be more strict about allowed chars in class name

spec/requests/embed_controller_spec.rb

@@ -100,7 +100,7 @@ describe EmbedController do
         expect(response.body).to match("data-referer=\"https://example.com/evil-trout\"")
       end
 
-      it "returns a list of topics" do
+      it "returns a list of top topics" do
         bad_topic = Fabricate(:topic)
         good_topic = Fabricate(:topic, like_count: 1000, posts_count: 100)
         TopTopic.refresh!
@@ -116,6 +116,16 @@ describe EmbedController do
         expect(response.body).to match("data-referer=\"https://example.com/evil-trout\"")
       end
 
+      it "wraps the list in a custom class" do
+        topic = Fabricate(:topic)
+        get '/embed/topics?discourse_embed_id=de-1234&embed_class=my-special-class', headers: {
+          'REFERER' => 'https://example.com/evil-trout'
+        }
+        expect(response.status).to eq(200)
+        expect(response.headers['X-Frame-Options']).to be_nil
+        expect(response.body).to match("class='topics-list my-special-class'")
+      end
+
       it "returns no referer if not supplied" do
         get '/embed/topics?discourse_embed_id=de-1234'
         expect(response.status).to eq(200)