Shows a list of online users

I don't think this will work if you have a user in the DB. Instead, I'd do this:

```ruby
add_column :users, :last_seen_at, :datetime, null: true

update("UPDATE users SET last_seen_at = updated_at")

change_column_null :users, :last_seen_at, false
```

That allows nulls, then backfills the column, then disallows nulls again.