add checkmark to the narbar of all constructed HTML

We added a checkmark all narbar's that has the make effect of
clicking on the **Nu HTML check this web page** link at the very
bottom.  This makes it much more convenient when one needs
to quickly check the HTML of a number of web pages.

Run `make www` to perform the above.

inc/navbar.default.html

@@ -7,6 +7,7 @@
   <a class="Right" href="%%PROG_LINK%%">%%PROG_TEXT%%</a>
   <a class="Right" href="%%MAKEFILE_LINK%%">%%MAKEFILE_TEXT%%</a>
   <a class="Right" href="%%INVENTORY_LINK%%">%%INVENTORY_TEXT%%</a>
+  <a class="Right" href="https://validator.w3.org/nu/?doc=%%VALIDATOR_ENCODED_URL%%">&#x2713;</a>
 </div>
 
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