Embed Notice

@julian @pfefferle there's a few options on what it would ideally be, but it *can* be any Object

1) group by `id`
2) possibly moderated by `attributedTo`
3) should have some collection associated with it somehow, just a flat list of everything included

the jury's out on how best to do #3 because the goal is to eventually be able to Follow a conversation. but https://github.com/w3c/activitypub/issues/486 makes it problematic to try to Follow a Collection directly...