[Glitch] Replace deprecated String.prototype.substr()

Port 0ec695e036 to glitch-soc

Signed-off-by: Tobias Speicher <rootcommander@gmail.com>
Signed-off-by: Claire <claire.github-309c@sitedethib.com>

app/javascript/flavours/glitch/features/video/index.js

@@ -90,7 +90,7 @@ 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