Issue Notes: record assignee changes in the comment timeline

Created by: PixnBits

as proposed in http://feedback.gitlab.com/forums/176466-general/suggestions/5168130-record-assignee-changes-in-the-comment-timeline

app/models/note.rb

     }, without_protection: true)
   end
 
+  def self.create_assignee_change_note(noteable, project, author, assignee, source)
+    body = "_Reassigned to @#{assignee.username}_"
+
+    create({
+      noteable: noteable,
+      project: project,
+      author: author,
+      note: body,
+      system: true
+    }, without_protection: true)
+  end
+
   # +noteable+ was referenced from +mentioner+, by including GFM in either +mentioner+'s description or an associated Note.
   # Create a system Note associated with +noteable+ with a GFM back-reference to +mentioner+.
   def self.create_cross_reference_note(noteable, mentioner, author, project)

app/observers/issue_observer.rb

   def after_update(issue)
     if issue.is_being_reassigned?
       notification.reassigned_issue(issue, current_user)
+      Note.create_assignee_change_note(issue, issue.project, current_user, issue.assignee, current_commit)
     end
 
     issue.notice_added_references(issue.project, current_user)