Replace deprecated String.prototype.substr()

String.prototype.substr() is deprecated (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr) so we replace it with startsWith() or endsWith()
Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

core_js/tools.js

@@ -288,7 +288,7 @@ function decodeURL(url) {
     }
 
     // Required (e.g., to fix https://github.com/ClearURLs/Addon/issues/71)
-    if(rtn.substr(0, 4) !== 'http') {
+    if(!rtn.startsWith('http')) {
         rtn = 'http://'+rtn
     }