Add rake task to regerate derivatives

lib/tasks/banner_images.rake

@@ -0,0 +1,19 @@
+namespace :banner_images do
+  desc "Generates derivates for banner images"
+  task generate_derivatives: :environment do
+    BannerImage.find_each do |banner_image|
+      attacher = banner_image.image_attacher
+
+      next unless attacher.stored?
+
+      attacher.create_derivatives
+
+      begin
+        attacher.atomic_persist            # persist changes if attachment has not changed in the meantime
+      rescue Shrine::AttachmentChanged,    # attachment has changed
+             ActiveRecord::RecordNotFound  # record has been deleted
+        attacher.delete_derivatives        # delete now orphaned derivatives
+      end
+    end
+  end
+end