Add action for plugins to load other API resources

 Part of #4055
Perform core authentication at 0 priority

Makes it easier for plugins to override the core API authentication

Part of #4055
Load admin profile class when editing a user
Fix permissions typo

Part of #4055
Specify 2 decimal places for woocommerce_format_decimal()

Part of #4055
Remove schema link
Remove PUT/DELETE endpoints

Will be implemented in 2.2

Part of #4055
Add total sales property to products response
Normalize response data types

Part of #4055
Fix incorrect total results number for pagination headers
Add permissions for count endpoints
Add total customers count to sales report

Part of #4055
Merge pull request #4185 from maxrice/rest-api-fixes-issue-4055

Various fixes for REST API