refactor: replace deprecated String.prototype.substr()

[CommanderRoot]

String.prototype.substr() is deprecated so we replace it with String.prototype.slice() which works similarily but isn't deprecated. .substr() probably isn't going away anytime soon but the change is trivial so it doesn't hurt to do it.

[Athanasius]

This looks trivially correct.

I'm going to guess we could do with running the entire thing through a linter, not to mention upgrading/testing the various third-party bits that are used.