Add warning message about permalink URL length.

Author: BigBlueHat

playground/next/index.html

@@ -81,6 +81,9 @@ <h2 class="ui massive header">JSON-LD Playground</h2>
           <i class="icon copy"></i>
         </button>
       </div>
+      <div class="ui warning message" v-show="permalinkURL.length > 2048">
+        This link is longer than 2kb, and may not work.
+      </div>
     </div>
     <button id="keyboard-shortcuts" class="ui icon button" data-position="left center">
       <i class="icon keyboard"></i>