Add db migration for notifications
user
```ruby
class AddColumnsToNotification < ActiveRecord::Migration
  def change
    add_column :notifications, :action, :string
    add_column :notifications, :actor_id, :integer
    add_column :notifications, :solution_id, :integer
    add_column :notifications, :iteration_id, :integer
    add_index :notifications, :user_id
  end
end
```
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