Fix mptt issues by making views that change the mptt fields mutually exclusive

* mutual exclusivity is achieved by doing a select for update on the first Page object
* add_view and move_page are now mutually exclusive. Other will follow...
Make copy_page mutually exclusive as well
Removed the old commit_on_success annotations
Use update_wrapper for the mutually_exclusive decorator
Merge pull request #1 from pbs/issue_1634

Fix for django-cms issue 1634