Introduce `orderby=include` support for `get_terms()`.

Props wpsmith.
Fixes #23261.
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Make the Twenty Fifteen search form submit button screen reader accessible rather than hidden to all users. Fixes #30110. Props mattweibe, rianrietveld.
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Add a missing Oxford comma to the install screen. Fixes #30107. Props Ankit K Gupta.
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Add `wp_json_encode()`, a wrapper for `json_encode()` that ensures everything is converted to UTF-8.

Change all core calls from `json_encode()` to `wp_json_encode()`.

Fixes #28786.


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Remove UNIQUE key from 'slug' column of terms table.

Each slug is a unique and beautiful snowflake, but let's enforce that
uniqueness elsewhere.

Props hotchkissconsulting.
Fixes #22023.
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Update the root svn:ignore property to match .gitignores. Fixes #30134.

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Fix a PHPDoc typo for `wp_json_encode()`.

Props JustinSainton.

See #28786.

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git-svn-id: http://core.svn.wordpress.org/trunk@30058 1a063a9b-81f0-0310-95a4-ce76da25c4cd