Restore Combined Index View

[askvortsov1]

I was thinking back to the combined index view issue (reverted in 72d06ba), and I think there might be a solution: we could run it through visibility scoping. This was much more confusing before with events, but now, all we need is an EloquentBuilder instance for Flarum\Discussion\Discussion. Unfortunately the query available under $search->getQuery() is a QueryBuilder, but we can wrap it in an EloquentBuilder to get access: core does a similar thing in https://github.com/flarum/core/blob/master/src/Post/Access/ScopePostVisibility.php#L55.

It would probably be a fair bit slower since visibility scoping is applied twice, but it should make it possible to restore combined view functionality.