Move mixins and bootstrap css to base directory

app/assets/stylesheets/application.scss

@@ -11,12 +11,17 @@
  *= require cal-heatmap
 */
-@import "main/*";
+
+@import "base/variables";
+@import "base/mixins";
+@import "base/layout";
+
 /**
  * Customized Twitter bootstrap
  */
-@import 'gl_bootstrap';
+@import 'base/gl_variables';
+@import 'base/gl_bootstrap';
 /**
  * NProgress load bar css

app/assets/stylesheets/gl_bootstrap.scss → app/assets/stylesheets/base/gl_bootstrap.scss

@@ -2,7 +2,6 @@
  * Twitter bootstrap with GitLab customizations/additions
  *
  */
-@import "gl_variables";
 // Core variables and mixins
 @import "bootstrap/variables";