Inbox add/remove side effect, add to target remote collection locally

[wmurphyrd]

Mastodon will start sending Add/Remove in their groups functionality to federate the contents of the group's timeline.

We don't currently process any side effects for inbox add/remove, but we would want a way to record their addition to the collection and fetch that collection later.

Perhaps not as simple as just adding the given remote collection id to activity._meta.collections, as there's no api to fetch a collection from another instance out of the local database

mastodon/mastodon#19059