Restores Devise and ActiveAdmin in Answers::Engine

Mounts active admin in generated app
Adds to migrations for Devise and ActiveAdmin

<hr />

To generate `MyAnswers` app:

`git checkout fix/243_restores_admin_sync`
`bin/answers MyAnswers`
`cd MyAnswers`
`bin/rails s`
[open localhost:3000](http://localhost:3000)
[open localhost:3000/admin](http://localhost:3000/admin)

Login to your answers-engine powered application.

```
SETTING UP DEFAULT USER LOGIN
Enter admin email: New user created: admin@answers.gsa.io
Admin password has been set to: Mahalo43
    ============================================================================
    Your new Answers application is now installed and mounts at '/'
  ============================================================================
```