Create new stylesheets for optional builds and add them to Gruntfile tasks

scss/bootstrap-grid.scss

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+// Bootstrap Grid only
+//
+// Includes relevant variables and mixins for the regular (non-flexbox) grid
+// system, as well as the generated predefined classes (e.g., `.col-4-sm`).
+
+
+//
+// Variables
+//
+
+// Grid system
+//
+// Define your custom responsive grid.
+$grid-breakpoints: (
+  // Extra small screen / phone
+  xs: 0,
+  // Small screen / phone
+  sm: 34em,
+  // Medium screen / tablet
+  md: 48em,
+  // Large screen / desktop
+  lg: 62em,
+  // Extra large screen / wide desktop
+  xl: 75em
+) !default;
+
+// Number of columns in the grid.
+$grid-columns:               12 !default;
+
+// Padding between columns. Gets divided in half for the left and right.
+$grid-gutter-width:          1.5rem !default;
+
+
+// Container sizes
+//
+// Define the maximum width of `.container` for different screen sizes.
+
+$container-max-widths: (
+  sm: 34rem,    // 480
+  md: 45rem,    // 720
+  lg: 60rem,    // 960
+  xl: 72.25rem  // 1140
+) !default;
+
+
+//
+// Grid mixins
+//
+
+@import "variables";
+
+@import "mixins/clearfix";
+@import "mixins/breakpoints";
+@import "mixins/grid-framework";
+@import "mixins/grid";
+
+@import "grid";