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
I think because the audience of DevTips mostly aren't experienced programmers and a not that familiar with JavaScript, it's good to have a code that at least somewhat easy to map to English, that's why I went with two `if`s.

We want selection of active client to wrap around, so it's two possibilities:

1. If we are going past the right bound (`index > numberOfClients`), we want to return to the left bound (`index = 0`).
2. If we are going past the left bound (`index < 0`), we want to return to the right bound (`index = numberOfClients`).

I would advise against using double ternary, it forces you to create branching model in your head, which you don't want.