Prohibit changing licences in certain cases

Prohibit changing licence options in certain cases. This will be a checkbox in admin area. e.g. creative commons SA, should not be changeable by anyone else but the original author.

Others should not be able to edit a page and change the license except the site admin and original author unless specified otherwise by the admin. Another situation to consider is when one person sets up a book and wants contributors, but he wants to make sure the whole book is entirely under one license. Just like the license can now "trickle down" to child nodes, he should be able to say child nodes can't pick another license.