Make it possible for lecturers/admins to mark live streams/scheduled lectures as private

[YiranDuan721]

Is your feature request related to a problem? Please describe.
Currently it is possible to mark VoDs as private. I think in some cases one also wants to mark lectures without VoDs as private.
This can also be a solution to #1265, although not perfect.

[YiranDuan721]

#1340 makes sure that a private live/scheduled stream is not listed on home page and course page. And the status "Private" is also applicable to live/scheduled stream, with the current database structure, so this hopefully don't need a big change.

[feriel97]

@joschahenningsen was this left out on purpose? because the mark as private/mark as public option is implemented for VODs but not available for livestreams.

[joschahenningsen]

No, this is not intended and I'd consider it a bug.

[feriel97]

I would like to take a look at it and try to fix it. Can you also assign it to me?