Add invisible class

• Adds an invisible class, which has visibility: hidden for hiding but keeping the space on the page.

src/scss/6_utilities/_utilities.scss

@@ -9,4 +9,8 @@
 
 .hidden {
     display: none !important;
+}
+
+.invisible {
+    visibility: hidden !important;
 }
\ No newline at end of file