Replace String.prototype.substr() occurrences with String.prototype.substring()

As outlined in https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr, which refers to the ECMA-262 specification, using the `substr` function is advised against.

Hence this PR, which replaces all remaining `substr` occurrences with `substring` instead. Please refer to https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr#Syntax respectively https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substring#Syntax for the differences between the two functions.

Note that in most cases in the code-base there's only one argument passed to `substr`, and those require no other changes except replacing "substr" with "substring". For the other cases, the `substr(start, length)` calls are changed to `substring(start, start + length)` instead.

web/ui_utils.js

@@ -537,7 +537,7 @@ function isDataSchema(url) {
   while (i < ii && url[i].trim() === '') {
     i++;
   }
-  return url.substr(i, 5).toLowerCase() === 'data:';
+  return url.substring(i, i + 5).toLowerCase() === 'data:';
 }
 
 /**