Revamp grid classes to require .col base class and improve flexbox grid classes with auto layout via flex: 1

scss/_grid.scss

@@ -36,6 +36,18 @@
 // Common styles for small and large grid columns
 
 @if $enable-grid-classes {
+  .col {
+    position: relative;
+    min-height: 1px;
+    padding-right: ($grid-gutter-width / 2);
+    padding-left: ($grid-gutter-width / 2);
+
+    // Allow `.col` to use an automatic, even width when flex mode is enabled
+    @if $enable-flex {
+      flex: 1;
+    }
+  }
+
   @include make-grid-columns();
 }