Theme Installer: Fix displaying of errors.

props SergeyBiryukov.
fixes #27640.

Built from https://develop.svn.wordpress.org/trunk@27958


git-svn-id: http://core.svn.wordpress.org/trunk@27788 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Theme Installer: Better responsive styles for feature filters.

props matveb.
fixes #27661.

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git-svn-id: http://core.svn.wordpress.org/trunk@27789 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Revert the changes made to `data.title` in playlist Underscore templates in [27869]. `{{ data.title }}` causes the title to be double-encoded on the front end.

See #27574.


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git-svn-id: http://core.svn.wordpress.org/trunk@27790 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Theme Installer: Use JSONP for api.wordpress.org requests.

fixes #27639.

Built from https://develop.svn.wordpress.org/trunk@27961


git-svn-id: http://core.svn.wordpress.org/trunk@27791 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Theme Installer: Don't encode description entities/HTML.

props johnbillion.
fixes #27641.

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git-svn-id: http://core.svn.wordpress.org/trunk@27792 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Theme Installer: Separate API from an event handler to avoid issues where a false return value stops the event.

see #27055.

Built from https://develop.svn.wordpress.org/trunk@27963


git-svn-id: http://core.svn.wordpress.org/trunk@27793 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Better checks for contributors when saving posts.

props dd32, kovshenin, plocha.
see #27452.

Built from https://develop.svn.wordpress.org/trunk@27964


git-svn-id: http://core.svn.wordpress.org/trunk@27794 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Inline documentation for remaining hooks in wp-admin/includes/upgrade.php.

Props jackreichert, vancoder.
Fixes #27646.

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git-svn-id: http://core.svn.wordpress.org/trunk@27795 1a063a9b-81f0-0310-95a4-ce76da25c4cd