course: provide a way to temporarily disable shared project

[slel]

It can be useful to disable a course's shared project for a while.

This can prevent students from accidentally starting it instead of their own project, for instance during an exam.

Currently

• one can delete an existing shared project; one can then create a fresh shared project again, but not recover the deleted one;
• one can remove students manually, one by one, from a course's shared project, but they get re-added automatically if one opens the .course file.

It would be nice to

• either be able to un-delete a course's shared project
• or to have a way to remove all course students from it and to re-add them later

[williamstein]

We could also implement a new primitive for projects called "Disabled", and only people with a special elevated privilege can toggle the disabled state. When disabled, the project doesn't run and no files are accessible.

Thoughts?

[slel]

That would work. (It would also be nice to be able to make a project readonly: files could be read and copied to other projects, but not created, deleted or edited. )