Replace deprecated String.prototype.substr()

[CommanderRoot]

Description

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.

Caveats

None

Tests

Test plan

Tested on

• [ ] Windows/Thunderbird
• [ ] Linux/Thunderbird
• [ ] macOS/Thunderbird

Check list

• [ ] Code is properly documented
• [ ] Code is properly formatted
• [x] Commits have been squashed
• [ ] Documentation (manuals or wiki) has been updated or is not required

Reviewers