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.


git-svn-id: https://develop.svn.wordpress.org/trunk@30561 602fd350-edb4-49c9-b593-d223f7449a82
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.

git-svn-id: https://develop.svn.wordpress.org/trunk@30562 602fd350-edb4-49c9-b593-d223f7449a82