Show warning about inaccessible activities to course admins on course show

[chvp]

I opted not to add a quicklink to fix the situation, since the exercises could be in multiple repositories. If we really want to, we could add a quicklink if they're only from one repository, but that would make things more complex.

• Tests were added

Closes #3135.

[bmesuere]

I agree with not adding the quicklink, but the current error message doesn't really help the user. It provides no hint at how to fix this. Can we be more helpful? Some ideas:

• Link to a docs page explaining private exercises and access management. (I don't know if we have a page like that)
• Can we point them in the right direction of whom to request access to? (Not super easy I think?)
• Add a link to the feedback form so we can help them

[chvp]

I agree with not adding the quicklink, but the current error message doesn't really help the user. It provides no hint at how to fix this. Can we be more helpful? Some ideas:

• Link to a docs page explaining private exercises and access management. (I don't know if we have a page like that)

We don't.

• Can we point them in the right direction of whom to request access to? (Not super easy I think?)

As hard as quicklinking the fix.

• Add a link to the feedback form so we can help them

Sure, I'll do that.