Support column sanitisation in `partially_delete` clauses

[urbanautomaton]

[This isn't for immediate action - I'm just making a note here so I can follow up later.]

The gem supports a partially_delete option for tables, in which a subset of the table is deleted and the rest retained. The DSL block takes an allow call, and columns are validated in the usual way (i.e. all columns must be explicitly allowed):

partially_delete 'notifications' do
  where notifiable_type: 'Mention'
  allow 'id', 'user_id', 'notifiable_id', 'notifiable_type', 'read_at', 'created_at', 'grouped', 'group_id'
end

Because this block doesn't take a sanitise call, however, it means that partially-deleted tables must have all their columns whitelisted. It might be nice to support delete-then-sanitise. It's taking longer and longer to sanitise the staging DB, so we're likely to want to work with table subsets more in the future.