Re-organize SASS structure

[Ismail-elkorchi]

Changes proposed in this Pull Request:

This pull request builds on top of the amazing work done by @samikeijonen in https://github.com/Automattic/_s/pull/1232 and https://github.com/Automattic/_s/pull/1234.

Adding to this the great feedback gotten on https://github.com/Automattic/_s/issues/1215 resulted in this new structure:

# Generic
    - Normalize
    - Box sizing
# Base
    - Typography
    - Elements
    - Links
    - Forms
## Layouts
# Components
    - Navigation
    - Posts and pages
    - Comments
    - Widgets
    - Media
    - Captions
    - Galleries
# plugins
    - Jetpack infinite scroll
# Utilities
    - Accessibility
    - Alignments

This PR share with https://github.com/Automattic/_s/pull/1232 and https://github.com/Automattic/_s/pull/1234 the same goals :

• Simplify SASS folder structure.
• The main goal is to have more readable structure.
• SASS folder structure should be easily maintained.
• Structure should be inline with current best practises.
• Structure should be good starting point in the bigger projects also.

Related issue(s):

Fixes : https://github.com/Automattic/_s/issues/1215 Closes : https://github.com/Automattic/_s/pull/1232 Closes : https://github.com/Automattic/_s/pull/1234