MM-58577 Check remote ownership for posts and reactions (#27317)

* - ensure that posts and reactions can only be added via sync when coming from a remote that the target channel is shared with.
- ensure that posts and reactions are only modified/deleted by the remote that owns them.

* check that reaction belongs to post that belongs to channel that is shared with remote;  check that posts belong to channel shared with remote

* check for correct error type in unit test

* tweak unit test

server/platform/services/sharedchannel/util.go

@@ -148,10 +148,3 @@ func reducePostsSliceInCache(posts []*model.Post, cache map[string]*model.Post)
 	}
 	return reduced
 }
-
-func SafeString(p *string) string {
-	if p == nil {
-		return ""
-	}
-	return *p
-}