Refactor usages of z-index

[MeoMix]

My usages of z-index have gotten a bit messed up. It would be good to sit down and go over what is needed from top-to-bottom and get them completely ironed out and sensible. Right now I have to use !important to get what I need in a few spots.

For reference, the z-index declarations are located here: https://github.com/MeoMix/StreamusChromeExtension/blob/Development/src/less/utility.less

It would probably be prudent to move z-index into its own file, too. You can find a reference for how it should look here: https://medium.com/@fat/mediums-css-is-actually-pretty-fucking-good-b8e2a6c78b06