subscription model now has issues through shipped_issues

In a similar vein to https://github.com/nebulab/spree-subscriptions/pull/36, I find it very convenient to be able to write `subscription.issues` rather than something like `Spree::Issue.where(id: subscription.shipped_issues.pluck(:issue_id))` or `subscription.shipped_issues.collect {|shipped_issue| shipped_issue.issue}`. Seems like it'd be useful to other folks as well.