PERF: Don't serialize value for theme_fields unnecessarily (#21201)

The value field of ThemeField is only used when viewing a diff in the staff action logs and local theme editing. value is being serialized into the theme index as well, which is not used. It's a huge amount of JSON that we can cut by removing it.

This also breaks up the various theme serializers into separate classes so they autoload properly (or at least restart the server on edit)

spec/system/admin_customize_themes_spec.rb

@@ -27,4 +27,16 @@ describe "Admin Customize Themes", type: :system, js: true do
       expect(theme.reload.color_scheme_id).to eq(color_scheme.id)
     end
   end
+
+  describe "when editing a local theme" do
+    it "The saved value is present in the editor" do
+      theme.set_field(target: "common", name: "head_tag", value: "console.log('test')", type_id: 0)
+      theme.save!
+
+      visit("/admin/customize/themes/#{theme.id}/common/head_tag/edit")
+
+      ace_content = find(".ace_content")
+      expect(ace_content.text).to eq("console.log('test')")
+    end
+  end
 end