Reorganize JS and SCSS folders.

This groups Controllers, Routes, and Views into folders based on the route name.

### Before

```
controllers/
    AController.js
    BController.js
    CController.js
routes/
    ARoute.js
    BRoute.js
    CRoute.js
views/
    AView.js
    BView.js
    CView.js
```

### After

```
apps/a/
    AController.js
    ARoute.js
    AView.js
apps/b/
    BController.js
    BRoute.js
    BView.js
apps/c/
    CController.js
    CRoute.js
    CView.js
```

A few example files are also added to encourage consistent naming of components, mixins, and helpers. This is what the default JS folder would look like.

```
apps
    application
        ApplicationController.js
        ApplicationRoute.js
        ApplicationView.js
    index
        IndexController.js
        IndexRoute.js
        IndexView.js
components
    RenameMeComponent.js
helpers
    rename-me.js
mixins
    RenamableMixin.js
model
    Rename.js
app.js
libs.js
router.js
```

While we are bikeshedding the folder structure, I thought it would make sense to revisit the scss folder not that we are using Ember.

The new structure tries to mirror the Ember structure as closely as possible.

Any styles for a specific route should go in the respective `apps/_route-name.scss` file.

Any styles for a specific component should go in `components/_component-name.scss`.

The `base/_body.scss`, `base/_header.scss`, and `base/_footer.scss` were all removed. Those rules should go in `apps/_application.scss`, or if you are using a component for the header or footer, `components/_site-header.scss`.

The `placeholders/_placeholder.scss` and `mixins/_mixins.scss` are moved to `helpers/`. I also added a `helpers/_colors.scss` and `helpers/_type.scss`. In general, any placeholders, mixins, or global variables belong in the `helpers` directory.

This is what the default folder would look like after running the generator.

```
apps
    _application.scss
    _index.scss
components
    _rename-me.scss
fonts
    _icons.scss
helpers
    _colors.scss
    _mixins.scss
    _type.scss
style.scss
tests.scss
```

This is what the `style.scss` file looks like after running the generator.

```scss
@import "red-sass/red-sass";
@include reset;

@import
"helpers/colors",
"helpers/type",
"helpers/mixins",

"fonts/icons",

"components/rename-me",

"apps/application",
"apps/index";
```