Make purchase helper pick default storage location

Changed the purchase helper to use the default storage location
of the organization first, before using any other storage locations
that are available.

Also added a (failing) spec to check that we can see the option
on the page. Should be fixed in the next commit

app/helpers/purchases_helper.rb

@@ -5,6 +5,6 @@ def purchased_from(purchase)
   end
 
   def new_purchase_default_location(purchase)
-    purchase.storage_location_id.presence || current_organization.intake_location
+    current_organization.default_storage_location || purchase.storage_location_id.presence || current_organization.intake_location
   end
 end

spec/requests/purchases_requests_spec.rb

@@ -1,5 +1,6 @@
 RSpec.describe "Purchases", type: :request do
   let(:organization) { create(:organization) }
+  let(:storage_location) { create(:storage_location, organization: organization) }
   let(:user) { create(:user, organization: organization) }
   let(:organization_admin) { create(:organization_admin, organization: organization) }
 
@@ -48,11 +49,13 @@
 
     describe "GET #new" do
       subject do
+        organization.update!(default_storage_location: storage_location)
         get new_purchase_path
         response
       end
 
       it { is_expected.to be_successful }
+      it { is_expected.to include(storage_location.name) }
     end
 
     describe "POST#create" do