Add globalizable tests for all translatable models

So now we test in depth at the model level, and can be a bit more
relaxed about integration tests for translations.

Note we're defining some extra factories to make sure all translatable
attributes with presence validation rules are mandatory. This way we can
simplify the way we obtain required fields, using `required_attribute?`.
Otherwise, fields having an `unless` condition in their presence
validation rules would count as mandatory even when they're not.

spec/models/site_customization/page_spec.rb

@@ -3,6 +3,8 @@ require "rails_helper"
 RSpec.describe SiteCustomization::Page, type: :model do
   let(:custom_page) { build(:site_customization_page) }
 
+  it_behaves_like "globalizable", :site_customization_page
+
   it "is valid" do
     expect(custom_page).to be_valid
   end