1) Icon optimizations
* Load styles for expand icon in
JavaScript since it relates to table of contents and that is only
enabled with JavaScript
* Drop vector-icon--x-small class since it only applies to this icon
* Remove some FIXMEs to an issue that has already been fixed
upstream
2) Move styles that do not need to be render blocking to skins.vector.js
* Move sticky header to skins.vector.js since it doesn't work without
JavaScript
* Move popupNotification to skins.vector.js since this is never server
side rendered.
Change-Id: I98e6c0826b514aaefcf88f66275bba37125129b1
