Reorganized CSS

Moved `scss/`, `img/`, and `css/` to root
Combined `img/` with `images/' and moved `images/` to root
Fixed font links and `jetpack-icons.css` paths
Gigantic CSS reorg

Features
* Atomic modular layout
* nested media queries (more maintainable)
* requires Sass 3.3 or higher to compile
Combined `_page-modules.scss` into `_settings.scss`
Moved `_mixins` folder and adjusted font variables
Concatenates front-end module styles

* concatenates most front-end styles into `jetpack.css`
* cssjanus also runs on `jetpack.css` adding rtl styles
Refactored grunt and added icon subfolders

* Would like to move Genericons folder into icons (needs discussion)
Adjust files concatenated for jetpack.css
Added new grunt task: `grunt concatenate`

* Manually concatenates front-end modules into one file
Added TODOs
Adding initial skeleton that will take care of imploding css files
Added filter to disable imploding css
Added dequeues
Added tests to `implode_frontend_css`.
Yoda conditions.
Removed leftover banner
Minor style changes
Minor style changes

Fixed a few things that were bugging me
* search icon now disappears on focus for module search
* feature hovers 100% better
* rocketeers no longer get cut off by the header
Increased repaint performance