Add basic unit tests for `is_plugin_active_for_network()`

Props rodrigosprimo
Fixes #30491

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git-svn-id: http://core.svn.wordpress.org/trunk@30548 1a063a9b-81f0-0310-95a4-ce76da25c4cd
TinyMCE: don't hide the image toolbar when the iframe window fires onresize. There is a bug in several browsers that triggers onresize when a tooltip is shown, only in RTL mode. Also use better variable name. Fixes #30147.
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git-svn-id: http://core.svn.wordpress.org/trunk@30549 1a063a9b-81f0-0310-95a4-ce76da25c4cd
When `json_encode()` returns a JSON string containing `'null'` in PHP 5.4 or earlier, `wp_json_encode()` will now sanity check the data, as older versions of PHP failed to encode non UTF-8 characters correctly, instead returning `'null'`.

Fixes #30471.

Built from https://develop.svn.wordpress.org/trunk@30561


git-svn-id: http://core.svn.wordpress.org/trunk@30550 1a063a9b-81f0-0310-95a4-ce76da25c4cd
TwentyFifteen:
- Fix image caption alignment in the editor.
- Remove unused var from functions.js.
Also includes few images that were shrunk a bit when running grunt precommit.
Fixes #30493.
Built from https://develop.svn.wordpress.org/trunk@30562


git-svn-id: http://core.svn.wordpress.org/trunk@30551 1a063a9b-81f0-0310-95a4-ce76da25c4cd