Use configurable adjustment promotion source types in Thumbnail.for

There's a View Component for rendering icons, and it's got a convenience
method for rendering adjustables to adjustments.

This uses the `Spree::Config.adjustment_promotion_source_types`
configuration option to figure out whether an adjustable needs a
megaphone as an icon.

With the legacy promotion system extracted, `Spree::Promotion` will not
necessarily be available.

admin/app/components/solidus_admin/ui/thumbnail/component.rb

@@ -31,7 +31,7 @@ def call
 
   def self.for(record, **attrs)
     case record
-    when Spree::PromotionAction then new(icon: "megaphone-line", **attrs)
+    when *Spree::Config.adjustment_promotion_source_types then new(icon: "megaphone-line", **attrs)
     when Spree::UnitCancel then new(icon: "close-circle-line", **attrs)
     when Spree::TaxRate then new(icon: "percent-line", **attrs)
     when Spree::LineItem then self.for(record.variant, **attrs)

admin/spec/components/previews/solidus_admin/ui/thumbnail/component_preview/overview.html.erb

@@ -52,7 +52,6 @@
   <% attachment = Object.new.tap { def _1.url(*) "https://placekitten.com/280/200"; end }  %>
   <% image = Spree::Image.new.tap { _1.define_singleton_method(:attachment) { attachment } } %>
   <% [
-    Spree::PromotionAction.new,
     Spree::UnitCancel.new,
     Spree::TaxRate.new,
     image,

core/spec/rails_helper.rb

@@ -10,6 +10,7 @@
   lib_name: 'solidus_core'
 )
 
+require 'solidus_admin'
 require 'rspec/rails'
 require 'rspec-activemodel-mocks'
 require 'database_cleaner'

legacy_promotions/spec/components/solidus_admin/ui/thumbnail/component_spec.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe SolidusAdmin::UI::Thumbnail::Component, type: :component do
+  describe ".for" do
+    let(:promotion_action) { Spree::PromotionAction.new }
+
+    subject { render_inline described_class.for(promotion_action) }
+
+    it "displays a megaphone" do
+      subject
+      expect(page).to have_xpath("//use[contains(@*, '#ri-megaphone')]")
+    end
+  end
+end

legacy_promotions/spec/rails_helper.rb

@@ -76,6 +76,10 @@
 
 Capybara.javascript_driver = (ENV['CAPYBARA_DRIVER'] || :selenium_chrome_headless).to_sym
 
+# VIEW COMPONENTS
+Rails.application.config.view_component.test_controller = "SolidusAdmin::BaseController"
+require "view_component/test_helpers"
+
 RSpec.configure do |config|
   config.fixture_path = File.join(__dir__, "fixtures")
 
@@ -101,6 +105,8 @@
     DummyApp.use_solidus_admin = false
   end
 
+  config.include ViewComponent::TestHelpers, type: :component
+
   config.include Spree::TestingSupport::JobHelpers
   config.include SolidusAdmin::TestingSupport::FeatureHelpers, type: :feature
   config.include FactoryBot::Syntax::Methods