Better LESS organization

**Looking for feedback on this new branch/PR**

Roots is already opinionated, so based on feedback from current users this may or may not get merged into master.

* Move Bootstrap core files to `assets/less/vendor/bootstrap/`
* New `_variables.less` file which is used to override Bootstrap variables and hold your custom variables (it's included directly after Bootstrap's variables file — Bootstrap is imported from a new `_bootstrap.less` file)
* New `_global.less` file for base styling and custom mixins
  * `.module()` mixin included that removes top and bottom margins
* New `assets/less/components/` directory
  * Placeholder components for buttons and forms (you'd add any other components in this directory and import them from `main.less`)
  * Gallery shortcode styling from `app.less` has been moved into the gallery component
  * WordPress generated classes styling has been moved into the wp-classes component. Expanded the `.alignleft` and `.alignright` classes to be mobile first (only float images if not on an extra small device like smartphones)
* New `assets/less/layouts/` directory
  * Included placeholder layouts for general styling, header styling, sidebar styling, footer styling, page styling, home page styling, and post styling
* New `main.less` file which imports Bootstrap, components, and layouts

I'm hoping that this setup would give you a better starting point for your project. Please let me know of any suggestions or feedback you have!

```
├── components
│   ├── _buttons.less
│   ├── _forms.less
│   ├── _gallery.less
│   └── _wp-classes.less
├── layouts
│   │── pages
│   │   └── _home.less
│   │── _footer.less
│   │── _general.less
│   │── _header.less
│   │── _pages.less
│   │── _posts.less
│   └── _sidebar.less
├── vendor
│   └── bootstrap
├── _bootstrap.less
├── _global.less
├── _variables.less
└── main.less
```