FEATURE: Staff members can lock posts

Locking a post prevents it from being edited. This is useful if the user
has posted something which has been edited out, and the staff members don't
want them to be able to edit it back in again.

db/migrate/20180125185717_add_locked_by_to_posts.rb

@@ -0,0 +1,5 @@
+class AddLockedByToPosts < ActiveRecord::Migration[5.1]
+  def change
+    add_column :posts, :locked_by_id, :integer
+  end
+end