4.9.8 breaks REST API attachment uploads on Multisite

[JakePT]

[43462] seems to have broken attachment uploads over the REST API in Multisite. This appears to be causing issues for people using Gutenberg:

​https://github.com/WordPress/gutenberg/issues/9134

​https://github.com/WordPress/gutenberg/issues/9142

When attempting to upload an image this error is occurring:

Call to undefined function upload_is_user_over_quota() in /PATH/TO/FOLDER/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php:808

The new check_upload_size() method of WP_REST_Attachments_Controller uses the upload_is_user_over_quota() function, however this function is part of wp-admin/includes/ms.php which is only loaded in the admin and doesn't appear to be loaded on REST requests.

[rachelbaker]

Require wp-admin/includes/ms.php

[rachelbaker]

In 43603:

REST API: Load missing required file for multisite users in WP_REST_Attachments_Controller::check_upload_size().

Requires wp-admin/includes/ms.php to make upload_is_user_over_quota() function available to multisite users uploading files.

Props JakePT, rachelbaker.
Fixes #44864.

[joachimschlosser]

The patch works for me and resolves the issue.

[pento]

[43603] was never ported to the 4.9 branch, let's put it in the 5.0 branch.

[danielbachhuber]

In 43691:

REST API: Load missing required file for multisite users

Requires wp-admin/includes/ms.php to make upload_is_user_over_quota() function available to multisite users uploading files.

Props JakePT, rachelbaker.
Merges [43603] to the 5.0 branch.
Fixes #44864.