get the converted post id with the 'go_subscriptions_converted_post_id' filter

* removed the restriction that only go-subscriptions can set read_post and read user caps

see https://github.com/GigaOM/legacy-pro/issues/4055
We're storing a post ID and checking for it, but what we really care about is a redirect URL. Should we filter for that instead?

```php
$redirect_url = apply_filters( 'go_subscriptions_thankyoucta', empty( $converted_post_id ) ? array( 'url' => home_url( '/' ), 'text' => 'Continue to home page' ) : array( 'url' => get_permalink( $converted_post_id ), 'text' => 'Continue to report' ), get_current_user_id() );