Cleaning up the MsUpload code

Perth

The MsUpload extension (which has nothing to do with more well-known MSs, Microsoft or multiple sclerosis) is a good extension, one I use quite a bit on some wikis. It has a slightly clunky interface, and every time I try to fix something in it I wish it's Javascript was slightly neater. So as a first step to that, I've set up eslint [1]. Maybe I'll lose interest now, and not come back and fix the actual errors... but at least it's a step towards something cleaner.