Only publishers should be able to unpublish

Currently users which have only contributor status can unpublish or archive an already published document.

This seems to be the case from one of the first commits (the second one if I count correctly):

https://github.com/xwiki-contrib/workflow-publication/commit/9fb54cfa3f6e184fa8f903b5a80ef1120ddc2e0a

While this might have been ok for pages that are published "in place" (ie. only validated without being published to the target page), it is not a good use case for target pages, as it allows contributors to remove content that has already been validated and published. Contributors should not be able to do that, same as they are not able e.g. to remove sections from that page without review. This should only be allowed for publishers.