While it has always been possible to initialize a user's roles and capabilities for another site than the current one in a multisite, the actual roles available were not switched prior to this change, possibly causing invalid roles to show up or actually valid capabilities not being available.
In order to fix this bug in a clean way, relevant parts of the `WP_User` class have been refactored. The ID of the site for which capabilities are currently initialized are now stored in a private property `WP_User::$site_id`. The `WP_User::for_blog( $blog_id )` and `WP_User::_init_caps( $cap_key )` methods have been deprecated in favor of `WP_User::for_site( $site_id )`. In addition, a new method `WP_User::get_site_id()` has been introduced to retrieve the site ID for which the user's capabilities are currently initialized.
Props ryanduff, jeremyfelt, flixos90.
Fixes#36961.
git-svn-id: https://develop.svn.wordpress.org/trunk@41624 602fd350-edb4-49c9-b593-d223f7449a82
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