adding show/hide helper classes/placeholders

opentech/static_src/src/sass/base/_base.scss

@@ -61,3 +61,27 @@ ol {
     margin: 0;
     list-style: none;
 }
+
+.is-invisible,
+%is-invisible {
+    z-index: -1;
+    opacity: 0;
+    transition: opacity, z-index, $transition;
+}
+
+.is-visible,
+%is-visible {
+    z-index: 5;
+    opacity: 1;
+    transition: opacity, z-index, $transition;
+}
+
+.is-hidden,
+%is-hidden {
+    display: none;
+}
+
+.is-unhidden,
+%is-unhidden {
+    display: block;
+}