Print multiple orders at once from orders dashboard

Firstly, I'd like to start by saying apologies for the messy commits - I have done my best with Git & GitHub and it's still something i'm massively trying to get used to. I've tried to merge this commits into one, I can't seem to do it no matter how many different methods i've tried, still a big learning curve for me.

Initially with this method I was going to use the existing `orders.js` file, however in trialling this I found it meant I needed to load a lot of other unnecessary scripts. So instead I opted to creating a `print-orders.js` which I believe is loading correctly and in fitting with the rest of the system.

I opted out of creating another ajax endpoint and simply used the existing one but modifying it to accept either a numeric string or an array.

I am unsure as to whether I was correct in using `array_values()` as opposed to incrementing a variable, but I felt this was a cleaner approach to keep track of the arrays position.

I'd love to hear feedback and if there is anyway I can improve this code, or if you have any suggestions for bettering my use of Github then please just let me know :)

Many thanks,
Warren