refs https://github.com/TryGhost/Team/issues/804
The associative table is used to implement the many-to-many relationship
between Products and Benefits. The `sort_order` column is needed because
a product's benefits should be orderable by an admin.
refs https://github.com/TryGhost/Team/issues/804
Benefits are tag-like resources which will be associated with Products.
The first iteration just requires a name for the benefit, which will be
stored as plaintext.
