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.



Built from https://develop.svn.wordpress.org/trunk@30388


git-svn-id: http://core.svn.wordpress.org/trunk@30385 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fourteen: replace ellipses appended to automatically generated excerpts with both ellipses and a "Continue Reading" link to meet accessibility-ready requirements.

Props davidakennedy, lancewillett. Fixes #30176.
Built from https://develop.svn.wordpress.org/trunk@30389


git-svn-id: http://core.svn.wordpress.org/trunk@30386 1a063a9b-81f0-0310-95a4-ce76da25c4cd