Splits the JSS/CSS code in core/js and core/css

Currently, all the JavaScript files and CSS files lie in one folder, a little more organization comes with this PR.

Let me know if you guys want to split segregate files further.

If merged, don't delete the branch, I will be pushing the User Management Frontend code here only.

@karlitschek @tanghus @MTGap @Raydiation @jancborchardt