There are now 3 places where we call out to the REST API from within wp-admin. This commit introduces a small library to allow overriding these calls, centralize nonce-passing logic, and eliminate the need to pass a full REST URL down to client code (this last feature is not yet used and will be explored in a separate ticket).
Fixes#40919.
Built from https://develop.svn.wordpress.org/trunk@41206
git-svn-id: http://core.svn.wordpress.org/trunk@41046 1a063a9b-81f0-0310-95a4-ce76da25c4cd
