Add missing methods to null promotion configuration

[mamhoff]

Summary

We've delegated a lot of configuration accessors from Spree::Config.instance to Spree::Config.promotions. When we switch the default configuration to be the Spree::Core::NullPromotionConfiguration (which does nothing), we need to have those setters in place, and we need them to issue deprecation warnings. This adds a configurable class that does nothing but issue many deprecation warnings, and adds the needed methods to Spree::Core::NullPromotionConfiguration.

For future readers: All of these deprecation warning will go away by using a non-stubbed promotion system, either the one in solidus_legacy_promotions or the one in the upcoming solidus_promotions.

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