DEV: Prevent videos from preloading metadata (#23807)

Preloading just metadata is not always respected by browsers, and
sometimes the whole video will be downloaded. This switches to using a
placeholder image for the video and only loads the video when the play
button is clicked.

spec/integration/blocked_hotlinked_media_spec.rb

@@ -48,7 +48,7 @@ RSpec.describe "hotlinked media blocking" do
       post = Fabricate(:post, raw: "![alt text|video](#{hotlinked_url})")
       expect(post.cooked).not_to have_tag("video source[src]")
       expect(post.cooked).to have_tag(
-        "video source",
+        "div",
         with: {
           PrettyText::BLOCKED_HOTLINKED_SRC_ATTR => hotlinked_url,
         },