Reorganize stylesheets and global styles

Simplifies the grunt watch task for sass. Renames sass partials with
leading underscores. Flattens stylesheet directory. The only remaining
raw css file is options.css.

Move globally-relevant styles to _global. Shrink overall font size.

Gruntfile.js

@@ -134,7 +134,7 @@ module.exports = function(grunt) {
         tasks: ['jshint']
       },
       sass: {
-        files: ['./stylesheets/**/!(manifest.css)'],
+        files: ['./stylesheets/*.scss'],
         tasks: ['sass']
       }
     },