Get rid of [unknown discussion] placeholder by completely hiding unknown discussions of DiscussionReferencedPost

[n-peugnet]

As @rob006 explained:

To be clear, I think that it would be much better to completely hide info about references in DiscussionReferencedPost, if they're not accessible. So if DiscussionReferencedPost has info about 2 discussions, but one of them is unavailable, then display info only about one available discussion. If all discussions in DiscussionReferencedPost are unavailable, then I would hide this event post completely. In this way we can avoid unnecessary noise and unintentional revealing of some information (currently you don't know title of discussion, but you still can see that specific person mentioned this discussion).

So if you decide to not have a setting to enable replacing unavailable discussion with placeholder, there will be no scenario with placeholder at all.

After thinking a little bit about it, I think I like this approach. But to implement it properly kind of requires #13 if we filter them in the frontend.
Another problem is that I don't know how to completely invalidate an EventPost in the frontend. Maybe I could override the view() method, but it would probably not look good. Maybe the best way would be to filter them from the backend, but not sure how feasible it would be.

Originally posted by @n-peugnet in #35 (comment)