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 handler. 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/0befb5976a749b4bfd55d41f3d101773f81be73b/assets/js/functions.js#L63-L100
Would using the conditional operator for here be better than both the if statements and have the following code for the index checking or would it just hinder readability?
index = index > numberOfClient ? index = 0 : index < 0 ? index = numberOfClients : index;