Move the assignment of roles to a package on package creation to an extension

[rgrp]

At the moment the assignment of user roles on package creation is done in setup_user_roles method in ckan/model/authz.py and is hard-coded.

This can be a pain to override (you can use a Plugin listening for package create events) and makes some things such as putting system in restricted mode more complex than it should be (see ticket:833).

An elegant solution would be to move this into an Extension this simplifies the code and make it easier for people override (just remove the default extension and plugin your own).

[dread]

This was done in #1025 through configuration. Happily you can setup config in an extension, so covers this ticket's aims too.