To account for it being displayed on custom order type Add/Edit screens
where the order type is not referred to as an 'Order' (e.g. a 'Subscription',
'Refund' or 'Pre-order').
* fixed indentation - swapped spaces for tabs at start of line, swapped
tabs for spaces min-line for alignment
* removed instances of `@return void` and `@access public`
* improved readability
@see http://make.wordpress.org/core/handbook/coding-standards/php/
- Removed post type classes. Since they were always loaded, there was
no benefit having them split up. Combined meaning less hooked in
methods.
- Stopped loading some function files and sessions in the admin area.
They are not needed there.
- Made a few admin page classes static as they are stateless