Twenty Fifteen: create consistent spacing with embeds and images.

Props afercia, fixes #30286


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git-svn-id: http://core.svn.wordpress.org/trunk@30302 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fifteen: simplify the code used to generate screen reader text for the is_home title.

Props davidakennedy, fixes #30305


Built from https://develop.svn.wordpress.org/trunk@30304


git-svn-id: http://core.svn.wordpress.org/trunk@30303 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fifteen: Minor formatting and docs tweaks, clearer variable name.

Props celloexpressions, see #29988


Built from https://develop.svn.wordpress.org/trunk@30305


git-svn-id: http://core.svn.wordpress.org/trunk@30304 1a063a9b-81f0-0310-95a4-ce76da25c4cd