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/public/model/reaction.go

@@ -64,3 +64,10 @@ func (o *Reaction) PreUpdate() {
 		o.RemoteId = NewString("")
 	}
 }
+
+func (o *Reaction) GetRemoteID() string {
+	if o.RemoteId == nil {
+		return ""
+	}
+	return *o.RemoteId
+}