Transfer widgets on room upgrade

[turt2live]

because this seems useful, even if no one has asked for it

[anoadragon453]

I suppose we don't want to transfer all of the events (adding/removing widgets over and over) but instead just those that haven't been paired with a deletion event (and thus those that are actually active in the room). Not sure if there's a quick way to do this or we just have to compare all the events.

[turt2live]

Should be a trivial thing to grab the state events and filter out the ones missing a url field (indicating deletion)

[anoadragon453]

TIL about current_state.

[anoadragon453]

So #5000 got pegged for now as transferring widgets on room upgrade may cause a lot of things to be broken silently, which is worse than just not doing it in the first place.