Refactoring VisibilityCopyJob and documenting

Reducing duplication of behavior to a consistent process. Improving
documentation.

The duplication was identified via `flay`

app/jobs/visibility_copy_job.rb

@@ -1,18 +1,28 @@
+# Responsible for copying the following attributes from the work to each file in the file_sets
+#
+# * visibility
+# * lease
+# * embargo
 class VisibilityCopyJob < ActiveJob::Base
+  # @api public
+  # @param [#file_sets, #visibility, #lease, #embargo] work - a Work model
   def perform(work)
     work.file_sets.each do |file|
       file.visibility = work.visibility # visibility must come first, because it can clear an embargo/lease
-      if work.lease
-        file.build_lease unless file.lease
-        file.lease.attributes = work.lease.attributes.except('id')
-        file.lease.save
-      end
-      if work.embargo
-        file.build_embargo unless file.embargo
-        file.embargo.attributes = work.embargo.attributes.except('id')
-        file.embargo.save
-      end
+      copy_visibility_modifier(work: work, file: file, modifier: :lease)
+      copy_visibility_modifier(work: work, file: file, modifier: :embargo)
       file.save!
     end
   end
+
+  private
+
+    def copy_visibility_modifier(work:, file:, modifier:)
+      work_modifier = work.public_send(modifier)
+      return unless work_modifier
+      file_modifier = file.public_send(modifier)
+      file.public_send("build_#{modifier}") unless file_modifier
+      file_modifier.attributes = work_modifier.attributes.except('id')
+      file_modifier.save
+    end
 end