[C2] Control changes in roles

[NicoDucou]

When changing a user role we should control if it is compatible with the assignation this user already have and show a message to indicate the problems that would happen.

For example if a user has a teacher role and we assign it a course A in which he is the teacher and we assign him as general coach of a session B. If we try to change the role of this user from teacher to student it should not allow us and show a message indicating the the student role is incompatible with the assignation as a teacher in course A and the assignation as general manager in session B and indicate that this should be change to be able to change the user role.

This is a specific example but we should verify the compatiblity for every change.

To be implemented in Chamilo 2