Newsletter module should allow for multiple users with separate mailing lists and types

[ss23]

From http://open.silverstripe.org/ticket/4447

Right now it seems that all the parts of the newsletter system is shared between the users that have access to the newsletter system.

For us, this is both unexpected and unwanted. The newsletter types, drafts and mailing lists should be tied to the user who created them and not be available globally to anyone who has access to the newsletter system.

The newsletter system should be able to accommodate a multi user scenario where only the templates are shared globally.