Tweak spoiler code

Refs #13, #14, flarum/core#2053.

extensions/markdown/less/forum.less

@@ -6,14 +6,22 @@
  */
 
 span.spoiler {
+  // Style the inline spoiler itself: a dark block, invisible font
   background: #444;
   color: transparent;
-  cursor: default;
+
+  // Try to generically hide all possible children
   * {
+    // Links and other interactive elements would give their presence
+    // away when hovering
     pointer-events: none;
-    padding: 0;
+
+    // Children can have special text or background colors
+    background: transparent;
     color: transparent;
   }
+
+  // Hide images visually
   img {
     visibility: hidden;
   }