Replace deprecated String.prototype.substr() (#17949)

* Replace deprecated String.prototype.substr()

.substr() is deprecated so we replace it with .slice() which works similarily but isn't deprecated

* Change String.prototype.substring() to String.prototype.slice()

.substring() and .slice() work very similary but .slice() is a bit faster and stricter

* Add ESLint rule to forbid usage of .substr and .substring

.substr() is deprecated and .substring() is very similar to .slice() so better to use .slice() at all times

Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

app/javascript/mastodon/features/video/index.js

@@ -91,7 +91,7 @@ export const fileNameFromURL = str => {
   const pathname = url.pathname;
   const index    = pathname.lastIndexOf('/');
 
-  return pathname.substring(index + 1);
+  return pathname.slice(index + 1);
 };
 
 export default @injectIntl