Improve localization for `WP_Date_Query` error strings.

* Don't create an extra variable for the string when it's only used once.
* Avoid using `<code>` HTML tags inside of translatable string.
* Improve data types and escaping on sprintfed values.

Props dd32.
Fixes #25834.
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git-svn-id: http://core.svn.wordpress.org/trunk@30299 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Correct grammar error in `WP_Date_Query` error message.

Props Offereins.
See #25834.
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Twenty Fifteen: simpler CSS by using divs instead of spans for menu descriptions.

Props philiparthurmoore, fixes #30296


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Twenty Fifteen: create consistent spacing with embeds and images.

Props afercia, fixes #30286


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Twenty Fifteen: simplify the code used to generate screen reader text for the is_home title.

Props davidakennedy, fixes #30305


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Twenty Fifteen: Minor formatting and docs tweaks, clearer variable name.

Props celloexpressions, see #29988


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Customizer: When navigating around the site within the Customizer preview, update the document title.

props westonruter.
fixes #28542.
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Customizer: Use `jQuery.fn.toggle()` instead of `slideUp`/`slideDown` if panel/section/control is not inserted into DOM yet.

jQuery does nothing when calling `slideUp` on elements that are not inserted into the DOM yet, which can now be the case now when first loading the Customizer as the panels, sections and controls get dynamically inserted, see #28709.

props westonruter.
fixes #30251.
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git-svn-id: http://core.svn.wordpress.org/trunk@30306 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Customizer: Trigger `widget-added` event when initializing.

Widget controls are now added to the pane dynamically via JavaScript, see #28709.
Remove the event trigger from `SidebarControl.addWidget()` as it's covered by `WidgetControl.ready()`.

props westonruter.
fixes #30236.
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git-svn-id: http://core.svn.wordpress.org/trunk@30307 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Customizer: Add support for the media library in Background Image controls.

`WP_Customize_Upload_Control` (PHP) and `api.UploadControl` (JS) can now open the media modal.

props celloexpressions, and ericlewis for docs.
see #21483.
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Always use a full filesystem path when including the FTP handler in PemFTP. This prevents other files in the includes directory being scanned/included accidentally.
See #29628

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PHP 5.2 compatibility for [30310], __DIR__ wasn't added until PHP 5.3. Props TobiasBg. Fixes #29628

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