* 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>