Improve the `doing_it_wrong()` text for `title-tag` theme support.

Props ocean90
Fixes #18548

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git-svn-id: http://core.svn.wordpress.org/trunk@30777 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Move the logic from WP_Image_Editor::get_quality() to WP_Image_Editor::set_quality(), so that 'wp_editor_set_quality' and 'jpeg_quality' filters run when setting the default value.

props markoheijnen.
fixes #29856.
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git-svn-id: http://core.svn.wordpress.org/trunk@30778 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fifteen: layout correction for IE8 and IE7 after r30388 introduced a defect with the header there.

Props iamtakashi, fixes #30630.


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Twenty Fifteen: moving to a single line for tranlator comments to prevent them being missed in i18n tools.

Props netweb, DrewAPicture, fixes #30603.


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git-svn-id: http://core.svn.wordpress.org/trunk@30780 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Revert [28948], which caused a regression.

see #29573 for trunk. see #28350.
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git-svn-id: http://core.svn.wordpress.org/trunk@30781 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Fix JSHint error introduced in [30780].

see #30608.
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Themes: Use Customizer deep-links inside the theme modal.

props DrewAPicture, ocean90.
see #25569, #25571.
fixes #30553.
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