Cap maximum width of video embeds

Embeds can end up taking up a lot of screen space in the single
annotation view and stream on desktop, so cap their width. Unfortunately
this has to be done in a slightly roundabout way due to the way that the
height of video embeds is set.

src/styles/sidebar/components/markdown-view.scss

@@ -29,5 +29,14 @@
 
   &__embed {
     width: 100%;
+
+    // Avoid embeds becoming too large when the annotation card is very wide.
+    // We can't just use `max-width` because the embed height is set using
+    // a CSS hack that sets `height` based on `width`.
+    //
+    // Therefore we cap the `width` based on the viewport size.
+    @media (min-width: 450px) {
+      width: 380px;
+    }
   }
 }