Feat(Promotions): Add helpers for checking activation options

There are three ways of activating a promotion, and they are mutually
exclusive. This adds a few helper methods so we know when a promotion
can have a code added, or promotion codes added, or whether it can
change the `apply_automatically` Boolean.

promotions/app/models/solidus_promotions/promotion.rb

@@ -138,6 +138,18 @@ def eligibility_results
       @eligibility_results ||= SolidusPromotions::EligibilityResults.new(self)
     end
 
+    def can_change_apply_automatically?
+      path.blank? && codes.empty?
+    end
+
+    def can_change_path?
+      !apply_automatically? && codes.empty?
+    end
+
+    def can_change_codes?
+      !apply_automatically? && path.blank?
+    end
+
     private
 
     def normalize_blank_values

promotions/spec/models/solidus_promotions/promotion_spec.rb

@@ -686,4 +686,82 @@
       expect(subject).to be_nil
     end
   end
+
+  describe "#can_change_apply_automatically?" do
+    subject { promotion.can_change_apply_automatically? }
+
+    let(:promotion) { create :solidus_promotion }
+
+    context "when the promotion has a path" do
+      before { promotion.path = "foo" }
+
+      it { is_expected.to be false }
+    end
+
+    context "when the promotion has a code" do
+      before { promotion.codes.new(value: "foo") }
+
+      it { is_expected.to be false }
+    end
+
+    context "when the promotion has neither a path nor a code" do
+      it { is_expected.to be true }
+    end
+  end
+
+  describe "#can_change_path?" do
+    subject { promotion.can_change_path? }
+
+    let(:promotion) { create :solidus_promotion }
+
+    context "when the promotion has a code" do
+      before { promotion.codes.new(value: "foo") }
+
+      it { is_expected.to be false }
+    end
+
+    context "when the promotion has a path" do
+      before { promotion.path = "foo" }
+
+      it { is_expected.to be true }
+    end
+
+    context "when the promotion has neither a path nor a code" do
+      it { is_expected.to be true }
+    end
+
+    context "when the promotion applies automatically" do
+      before { promotion.apply_automatically = true }
+
+      it { is_expected.to be false }
+    end
+  end
+
+  describe "#can_change_codes?" do
+    subject { promotion.can_change_codes? }
+
+    let(:promotion) { create :solidus_promotion }
+
+    context "when the promotion has a code" do
+      before { promotion.codes.new(value: "foo") }
+
+      it { is_expected.to be true }
+    end
+
+    context "when the promotion has a path" do
+      before { promotion.path = "foo" }
+
+      it { is_expected.to be false }
+    end
+
+    context "when the promotion has neither a path nor a code" do
+      it { is_expected.to be true }
+    end
+
+    context "when the promotion applies automatically" do
+      before { promotion.apply_automatically = true }
+
+      it { is_expected.to be false }
+    end
+  end
 end