Customizer: Don't trigger a change event if two unchanged object values are equal, second pass.

Make Underscore.js a dependency for `customize-base` and use `_.isEqual()` to compare the values.
(Underscore.js was already enqueued via wp-util.js for Widgets.)

props westonruter.
fixes #26061.
Built from https://develop.svn.wordpress.org/trunk@29907


git-svn-id: http://core.svn.wordpress.org/trunk@29661 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Comments: Don't print an empty HTML markup when `comment_reply_link()` returns no link.

props obenland.
fixes #29895.
Built from https://develop.svn.wordpress.org/trunk@29908


git-svn-id: http://core.svn.wordpress.org/trunk@29662 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fifteen: make font-size/line-height in editor-style.css the same as in style.css. Remove margins from the editor body, interferes with autoresize in some browsers and is overriden in the default styles. See #29799
Built from https://develop.svn.wordpress.org/trunk@29909


git-svn-id: http://core.svn.wordpress.org/trunk@29663 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fifteen: no `rem` in editor-style.css for now, see #29799
Built from https://develop.svn.wordpress.org/trunk@29910


git-svn-id: http://core.svn.wordpress.org/trunk@29664 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fifteen, editor-style.css: revert the font-size back to 19px and margin back to 83px, merge the two body selectors (don't seem needed), remove redundant rtl rules and fix body.rtl selector. See #29799
Built from https://develop.svn.wordpress.org/trunk@29911


git-svn-id: http://core.svn.wordpress.org/trunk@29665 1a063a9b-81f0-0310-95a4-ce76da25c4cd