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/52b08ad748858303e4f3ad91dd5918fc0dc65bd3/assets/js/functions.js#L63-L99