Symlink arrow-*-double to go-*-skip, add 24px go-*-skip

Summary:
The arrow-*-double icons were symlinked to the go-top/bottom/first/last icons. Those didn't look like double arrows, so the arrow-*-double icons are now symlinked to go-*-skip.

A previous commit with this change was reverted because there weren't any 24px go-*-skip icons, so this patch also includes the missing icons.

Test Plan:

Reviewers: VDG

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D21084