Administrative Contexts

Issue Summary

It would be nice if you could flag a context as 'administrative' where only certain role(s) having 'access administrative context' permission could see/edit them. For example, if you are using Context Assets and Context PHP- you may have certain contexts in use to build parts of a Drupal site that the end-user and/or client should not have access to.

Part of the reason I use Context is for block arrangement (super simple and better on performance) so, we do show end-user how to use them. It would be nice to hide contexts they should not see or potentially mess up.