CSS Refactor

Created by: tysongach

This PR is more improvements to Administrate’s stylesheets architecture. It’s a continuation of #347.

• Administrate’s stylesheets directory is now more organized and clear:
  • base – all styles for base HTML elements (no classes, id’s, etc.)
  • components – the majority of styles, these are all of our components that create the Administrate design
  • library – any non-rendering Sass, e.g. variables and mixins
  • application.scss – the main Sass manifest, which imports all of the individual Sass partials
• De-variablized some CSS value that didn’t gain much value from being a variable
• More closely aligned to thoughtbot’s SCSS style guide; running scss-lint is down to one warning now (which is an edge case)
• All changes in this PR are mostly structural and do little to change visual design
• Plus much more!