CB

This initially should just be for Emails to replace existing cbNotification API, but should likely support additional types (e.g. PM). Proposed structure as follows.

CBLib\Notification
CBLib\Notification\Email

Usage as follows.

Application::Notification()->send( to, from, subject, message )

Specify type in the constructor (e.g. Application::Notification( 'pm' )).

The problem is PMs are from a plugin while Emails are core. So it needs to be able to call plugin notification types without being dependent on those plugins (e.g. if notification type doesn't exist do nothing). This will likely rely on #5209 existing to call something like Application::PM->send( to, from, subject, message ) when using Application::Notification( 'pm' ).