refs https://github.com/TryGhost/Team/issues/946
This refactor pulls out the core logic so that we can easily add other
bulk operations without having to duplicate even more logic.
It also gives a consistent return value between bulk operations, renaming
`unsuccessfulIds` and `unsuccessfulRecords` to `unsuccessfulData`
We also add a bulkEdit method which will be used to bulk unsubscribe members
from the newsletter.
no issue
- we were only importing the `db` to access the `knex` instance, but
we can get this through the Bookshelf instance
- switches to pulling out `knex` from Bookshelf so we can remove the
remaining local require