Removes nicescroll from issuable sidebar
What does this MR do?
This is one of biggest performance issues when scrolling. Nicescroll is not so nice when it comes to scrolling. Every scroll there is reflows & repaints for the whole page which on larger issues/merge requests can cause some significant performance issues.
This replaces nicescroll with a little bit of a CSS hack. The main sidebar is overflow: hidden; whilst the inner element has the scroll with a width to push it way out of the box to make sure the scrollbar can not be seen (unless someone somehow has a scrollbar that is 100px wide).