seo: add json-ld structured data for videos.

Makes video posts eligible for rich results in Google, which means they
can include a thumbnail.

Ref: https://developers.google.com/search/docs/data-types/video

app/views/posts/show.html.erb

@@ -147,6 +147,10 @@
 
   <% if policy(@post).visible? %>
     <%= tag.meta property: "og:image", content: @post.open_graph_image_url %>
+
+    <% if @post.is_video? %>
+      <%= json_ld_video_data(@post) %>
+    <% end %>
   <% end %>
 
   <% if @post.twitter_card_supported? %>