Twenty Fifteen: adding context to page links for screenreaders.

Props afercia, fixes #30174.



git-svn-id: https://develop.svn.wordpress.org/trunk@30314 602fd350-edb4-49c9-b593-d223f7449a82
Twenty Fifteen: since color schemes can be customized let's call them base color schemes instead of just color schemes.

Props celloexpressions, iandstewart, fixes #30316



git-svn-id: https://develop.svn.wordpress.org/trunk@30315 602fd350-edb4-49c9-b593-d223f7449a82
Twenty Fifteen: adding context to pagination links for screenreaders.

Props afercia, iamtakashi, fixes #30152



git-svn-id: https://develop.svn.wordpress.org/trunk@30316 602fd350-edb4-49c9-b593-d223f7449a82
Twenty Fifteen: improve appearance of page links when there are tons of pages to link to

Props iamtakashi, fixes #30301



git-svn-id: https://develop.svn.wordpress.org/trunk@30317 602fd350-edb4-49c9-b593-d223f7449a82