Cache CommunityUser post counts

[cellio]

#1409 adds the "all communities" tab on the user profile, which shows that user's profiles on the rest of the network with some stats, currently just reputation and number of posts. Getting the number of posts per profile could get expensive at scale. Instead, we should cache this and update when posts are added or deleted.

We are already using counter_culture in a couple other places and could use it here to address this. We need to add a post_count counter to the CommunityUser model; CU doesn't have a direct relation to Post but can get there through User (has_many :posts, through: :user should do that, I think). The rest of the work is to add the counter_culture declaration and make it work for adding or deleting posts. That's the part I couldn't figure out, so we're spinning this off as a new issue.