Twenty Fifteen: Use a heading heirarchy to provide a better navigation experience when using screenreading software.

Props davidakennedy, rianrietveld, iamtakashi, afercia, davidakennedy, fixes #30065.

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git-svn-id: http://core.svn.wordpress.org/trunk@30072 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Adjust caching for `get_term_by()` calls:

* Remove md5 hashes for term name cache keys
* Remove the namespace for the keys for `names` and `slugs` and add them to the group names
* Remove `wp_get_last_changed()`, which @nacin hated
 
Props tollmanz.
Fixes #21760.

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git-svn-id: http://core.svn.wordpress.org/trunk@30073 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Introduce a new means of outputting a `<title>` tag in the theme head. Requires a theme to add support by calling `add_theme_support( 'title-tag' )`. This is the first step in adding a more robust means of generating and outputting the title tag.

See #18548.
Props obenland, chrisbliss18, joostdevalk.


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git-svn-id: http://core.svn.wordpress.org/trunk@30074 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Define `JSON_PRETTY_PRINT` so it can be used with `wp_json_encode()`

* `JSON_PRETTY_PRINT` was introduced in PHP 5.4
* Now you can use it with lower PHP versions, without a notice

fixes #30139
see #28786
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git-svn-id: http://core.svn.wordpress.org/trunk@30075 1a063a9b-81f0-0310-95a4-ce76da25c4cd
In `_wp_handle_upload()`, if `test_upload` is set to `false` in the array of dangerous overrides that the function allows, the only thing that happens when an upload fails is more potential breakage.

`$test_uploaded_file` lets is know if `$file['tmp_name']` exists, which allows to exit with an error, instead of continuing to attempt to move the file.

`$test_upload` override is now a noop.

Fixes #28208.

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git-svn-id: http://core.svn.wordpress.org/trunk@30076 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Update inline docs for [30052].

Fixes #23261.
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git-svn-id: http://core.svn.wordpress.org/trunk@30077 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Docs and code standards cleanup for [30055] (`wp_json_encode()` & friends)

fixes #28786
props TobiasBg
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git-svn-id: http://core.svn.wordpress.org/trunk@30078 1a063a9b-81f0-0310-95a4-ce76da25c4cd