Refactor JavaScript for Clients section.

It was a bit hard to follow you because of all the branching going on in the previous client/next client handlers. Hopefully, [this code][1] is a bit easier to read.

I also included a cleanup commit, I'm not sure it's necessary and can remove it for a cleaner diff.

[1]: https://github.com/cust0dian/Artists-Theme/blob/6e4c4843ce7d5152d69a7037a94bfc09895e0ac7/assets/js/functions.js#L63-L100
Technically, this check isn't needed, because [when called with a negative number][1], `.eq` will return items from the end, so `.eq(-1)` will return last item.

Therefore, when we would click on `.client-control-prev` while first client is active and, basically, call `setActiveClient(-1)`, it would wrap around automatically, but I'd rather be explicit and not rely on implementation of another function.

[1]: https://api.jquery.com/eq/#eq-indexFromEnd