refs https://github.com/TryGhost/Team/issues/1469
With multiple newsletters, members will now be able to subscribe to one or more newsletters on the site. Previously, the subscription to default newsletter for a member was controlled via a single boolean `subscribed` column on the member table.
This change allows mapping multiple newsletters to a member via new pivot table that stores relation between a member and newsletter.
- adds new `members_newsletters` pivot table
- update tests
