More Grunt improvements:

• Fixes CSS & JS admin folder path for BBP_RTL_CSS, BBP_LTR_CSS & BBP_JS in Gruntfile.js
• Removes Mint & Evergreen CSS (including RTL) to use Mint & Evergreen SCSS
• Adds _admin.scss, _mixins.scss, _variables.scss via (​/trunk/src/wp-admin/css/colors) to build our Mint & Evergreen admin themes
• Adds /includes/admin/styles/evergreen/colors.scss & /includes/admin/styles/mint/colors.scss via ​2288dcd5b5 MP6 GitHub Repo Changset
• Adds support in Gruntfile.js to build minified and RTL Mint & Evergreen CSS
• Adds grunt-sass ~0.10.0 to package.json to build Mint & Evergreen CSS from SCSS source
• Reinstates minified and SCRIPT_DEBUG support for minified Mint & Evergreen CSS
• Adds minified and SCRIPT_DEBUG support for admin javascript & frontend javascript

Props netweb. See #2542.