Issues related to closed accounts

[pkukkon]

When searching for users to be added as course staff, the search dialog also lists closed accounts.

When an account is closed, it should most likely be removed from courses' staff as well.

Furthermore, such courses would need to be saved so that Muikku can get a notification about them and adjust its list of course teachers accordingly.

[aviljakaine]

Moved first part to a separate issue #1551