Twenty Thirteen: replace ellipses appended to automatically generated excerpts with both ellipses and a "Continue Reading" link to meet accessibility-ready requirements.

Props davidakennedy, lancewillett. Fixes #30178.
Built from https://develop.svn.wordpress.org/trunk@30390


git-svn-id: http://core.svn.wordpress.org/trunk@30387 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fifteen: Check if we're in the customizer before we avoid outputting HTML for an empty tagline. Avoids an issue with a not-updating customizer preview of the tagline if you're going in with an empty tagline.

Props philiparthurmoore, celloexpressions, fixes #30358.


Built from https://develop.svn.wordpress.org/trunk@30391


git-svn-id: http://core.svn.wordpress.org/trunk@30388 1a063a9b-81f0-0310-95a4-ce76da25c4cd