Fix YouTube figures and add a .content-width CSS class

assets/styles/root/002_layout.css

@@ -180,7 +180,6 @@
 
   figure .youtube {
     overflow: hidden;
-    max-width: var(--content-width);
     width: 100%;
   }
 

assets/styles/root/099_utility.css

@@ -1,6 +1,8 @@
 @layer utility {
     .centered { text-align: center; }
 
+    .content-width { grid-column: content-start / content-end; }
+
     .float-right {
         float: right;
         justify-self: end;