* Move the legacy `allowed_themes` filter to `WP_Theme::get_allowed_on_network()`, where it will continue to filter themes allowed on the network.
* Add `network_allowed_themes` filter to `WP_Theme::get_allowed()` and pass `$blog_id` to provide context.
* Add `site_allowed_themes` filter to `WP_Theme::get_allowed_on_site()` and pass `$blog_id` to provide context.
Props pauldewouters, lamosty, michalzuber, dmsnell, johnnypea, rob.
Fixes#28436.
git-svn-id: https://develop.svn.wordpress.org/trunk@36366 602fd350-edb4-49c9-b593-d223f7449a82