Restore assets and views in new engine

This restores asset and view functionality in the newly generated app.  It is setup to use the layout within the engine (similar to ActiveAdmin/RailsAdmin, unlike Devise) by having engine controllers inherit from `Answers::ApplicationController`. I also took the liberty to remove any unused files throughout the repo.

@amoose and @adelevie , please take a look. Generate a new app instance and run the rails server:

```sh
./bin/answers MyAnswers
cd MyAnswers
./bin/rails s
```
The server should load the correct css:
```html
<link data-turbolinks-track="true" href="/assets/answers/formatting.css?body=1" media="all" rel="stylesheet" />
<link data-turbolinks-track="true" href="/assets/answers/theme.css?body=1" media="all" rel="stylesheet" />
<link data-turbolinks-track="true" href="/assets/application.css?body=1" media="all" rel="stylesheet" />
```
and js:
```html
<script data-turbolinks-track="true" src="/assets/jquery.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/jquery_ujs.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/turbolinks.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/answers.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/answers/Markdown.Extra.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/jquery-ui/core.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/jquery-ui/datepicker.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/jquery-ui/widget.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/jquery-ui/button.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/jquery-ui/mouse.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/jquery-ui/draggable.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/jquery-ui/position.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/jquery-ui/resizable.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/jquery-ui/dialog.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/jquery-ui/sortable.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/active_admin/base.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/active_admin/lib/batch_actions.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/active_admin/lib/checkbox-toggler.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/active_admin/lib/dropdown-menu.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/active_admin/lib/flash.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/active_admin/lib/has_many.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/active_admin/lib/modal_dialog.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/active_admin/lib/popover.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/active_admin/lib/table-checkbox-toggler.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/active_admin/ext/jquery-ui.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/active_admin/ext/jquery.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/active_admin/application.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/answers/pagedown/Markdown.Converter.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/answers/pagedown/Markdown.Editor.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/answers/pagedown/Markdown.Sanitizer.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/answers/active_admin.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/answers/pagedown/demo/node/demo.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/answers/pagedown/local/Markdown.local.fr.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/answers/pagedown/node-pagedown.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/answers/skip_nav.js?body=1"></script>
<script data-turbolinks-track="true" src="/assets/application.js?body=1"></script>
```