Insert new designs at the end

This is not behind a feature flag, to ensure any newly created designs
will have a position set.

app/models/design_management/design_collection.rb

@@ -12,7 +12,9 @@ module DesignManagement
 
     def find_or_create_design!(filename:)
       designs.find { |design| design.filename == filename } ||
-        designs.safe_find_or_create_by!(project: project, filename: filename)
+        designs.safe_find_or_create_by!(project: project, filename: filename) do |design|
+          design.move_to_end
+        end
     end
 
     def versions

spec/models/design_management/design_collection_spec.rb

@@ -34,6 +34,13 @@ RSpec.describe DesignManagement::DesignCollection do
         collection.find_or_create_design!(filename: 'world.jpg')
       end.not_to exceed_query_limit(1)
     end
+
+    it 'inserts the design after any existing designs' do
+      design1 = collection.find_or_create_design!(filename: 'design1.jpg')
+      design2 = collection.find_or_create_design!(filename: 'design2.jpg')
+
+      expect(design1.relative_position).to be < design2.relative_position
+    end
   end
 
   describe "#versions" do