Show recipient users in admin list view for Activity.
Create a new Activity every time a post is shared with a new Space or
un-shared with a Space (corresponds to 'Post through sites' on screen,
but it's actually Spaces that are involved, not Sites. With our data at
the moment this is one-to-one, but this is not enforced (Space <- Feed
-> Site).
https://trello.com/c/dckCKX0x/