text: Title: App with custom data models doesn't import the app package
  Body: Version 2.5.2
  
  Noticed that when trying to upgrade using a migration that adds a custom data type (something that subclasses `TypeDecorator`) the migration script that gets created correctly generates the data model (e.g. `sa.Column('mytype', app.models.CustomType())`); however, it fails to import `app` at the top of the script, and thus raises `NameError: name 'app' is not defined` when you run it.
  
  Simple solution is to import the app.