Twenty Fifteen: Making the sidebar sticky for everyone. When we have a long sidebar, let it scroll with the content, but fixing the sidebar and no longer scrolling when we get to the end of the sidebar content. Scroll up and the sidebar starts scrolling up to, eventually staying fixed when it gets back to the top.

Props celloexpressions, avryl, fixes #30366.




git-svn-id: https://develop.svn.wordpress.org/trunk@30388 602fd350-edb4-49c9-b593-d223f7449a82