Only send subscription emails when subscribed is true

We were sending email notifications to all users for which a subscription existed, regardless of whether `subscribed` was true.
Two changes:

1. We should have a `subscribed` scope on `Subscription`
2. You can make Active Record do a subquery by doing this:

```ruby
User.where(id: subscriptions.where(subscribed: true).select(:user_id))
```

Alternatively, there might be a way to do this by applying a condition to `has_many`. I haven't run this code, but it might look something like this:

```ruby
has_many :subscribers, -> { where(subscriptions: {subscribed: true}) }, through: :subscriptions, :source: 'user'
```