Multisite: ensure that user_ids only work for users of this site

[mattwiebe]

When browsing Followers, they can have a user_id on their own instance that Mastodon clients then try to retrieve from their own instance. Eg say that a user is followed by my mattwiebe@mastodon.onlinne account. When the client queries the mastodon.online instance, it gets this:

{
  "id": "110770996307846062",
  "username": "mattwiebe",
  "acct": "mattwiebe",
  "display_name": "mw.social"
  // snip
}

But when browsing profiles, some clients (Ivory in my case) tries to query that ID against the current instance, doing api/v1/accounts/110770996307846062 against EMA. For wpcom, that particular ID is above our highest user_id, but many will produce a false positive for a user in the network who isn't even a member of the blog.

This doesn't fix for user ID collisions where the user ID from a remote user is the same as one that is also a member of the blog, but it's a good start.