use local copy of license button

Author: bodewig

assets/public-domain_0.png

@@ -129,7 +129,7 @@ <h1>Tossca</h1>
                   <p xmlns:dct="http://purl.org/dc/terms/" xmlns:vcard="http://www.w3.org/2001/vcard-rdf/3.0#">
                     <a rel="license"
                        href="https://creativecommons.org/publicdomain/zero/1.0/">
-                      <img src="https://i.creativecommons.org/p/zero/1.0/88x31.png" style="width: 88px;" alt="CC0"/>
+                      <img src="assets/public-domain_0.png" style="width: 88px;" alt="CC0"/>
                     </a>
                     <br />
                     To the extent possible under law,