Add active tabs

This is a bug fix and a feature.

In here are two fixes. The first is that I added the missing active tabs for `:adjustments, :customer_details, :refund_reasons, :reimbursement_types, :return_authorization_reasons, and :promotion_categories`. The second fix was f587e65 where the product sub menu for new taxonomies was not loading. It loads now.

The feature is that the tabs were abstracted into the `Spree::BackendConfiguration` model. This means that you do not have to overwrite the tabs file to add new active tabs. If you're creating an extension, you can instead perform `CONFIGURATION_TABS << :active_shipping_settings` and the memoization of the constants will ensure that `CONFIGURATION_TABS` does not lose `:active_shipping_settings` as one of its values. Also, the memoization would prevent reassignment of constants in concurrent environments (i.e. when you're using Sidekiq or Puma).

If this PR is accepted, then we should add that to the spree extensions developer guide so that users are aware.