Remove duplication in permission handling:

Remove duplication in permission handling:
* Use execute() as a Template Method, with permission() and handle() as the hook methods.
* Provide a default implementation for the permission() method, which checks for self.permissionType, if the attribute is defined.
* New hasPermission() method which checks whether the current user has a permission on the current class

editItemPermission method:
* Better error message when user is editing roles when they shouldn't be.
* Extracted isEditingSelf(), which checks whether a user is editing his/her own details.
* Use hasPermission method.

git-svn-id: http://svn.roundup-tracker.org/svnroot/roundup/trunk@2086 57a73879-2fb5-44c3-a270-3262357dd7e2