Give defaults for options on product Importer
remove broken variant "deleted" scope

this "deleted" scope doesn't remove the default "deleted_at: nil" condition so it just produces a query like this:

```sql
SELECT "spree_variants".*
FROM "spree_variants"
WHERE "spree_variants"."deleted_at" IS NULL
AND   "spree_variants"."deleted_at" IS NOT NULL
```

instead you have to use the "only_deleted" scope from the acts_as_paranoid gem
see https://github.com/goncalossilva/acts_as_paranoid/blob/ae0fe74/README.markdown#filtering

example: https://github.com/spree/spree/pull/4562

Fixes #4601
Support existing credit card feature on api#checkout

about #4598
provide consistent sort order for promotions

make pagination sane for promotions.
especially true for postgres which seems to do more to keep its promise that "If sorting is not chosen, the rows will be returned in an unspecified order".
since there are no indexes on this table we just use the primary key with most recent first.

Fixes #4596
Fix thank you message on order completion

Thank you message was no more displayed since it was using a wrong
params to test when order was just completed.
This solve the issue by using flash[:commerce_tracking] param
to test when to display the message. An helper method has been
added to aviod code duplication.
Use a more relavant naming and avoid using useless
'nothing special' string as content of the flash message.

Fixes #4586