Migrate styles

[beechnut]

One of the things I'm noticing is that the link to the styles CSS is the only broken link in the site right now:

Check HTML Links
  📖 Found 1051 files containing
    🔗 46998 internal links
  👀 Checking...
  ❌ Found 1 missing reference targets (used by 957 links) while checking 1051 files:
    1. missing reference target _site/assets/css/styles.css
      from _site/2014/03/12/coming-soon/index.html:60:8 via href="/assets/css/styles.css"
      from _site/2014/03/19/hello-world-we-are-18f/index.html:60:8 via href="/assets/css/styles.css"
      from _site/2014/03/21/29-minutes/index.html:60:8 via href="/assets/css/styles.css"
      ... 954 more references to this target

Acceptance criteria

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1. Styles are correct, and showing the equivalent of what we see on 18f.gsa.gov (prod)

[jasnakai]

Consolidted the styles so that they visually match the live 18F website. Moved the relevant files from _sass to the assets/common/styles folder. Deleted the duplicate/unnecessary files and the _sass folder. Removed references to the _sass folder.
The CSS could use a cleanup since the styles from two sites were combined. I will do what I can but not get caught up on it.
Branch name: jn/fix-styles-link