Create toast JS plugin, add unit tests.

scss/_toasts.scss

@@ -1,4 +1,5 @@
 .toast {
+  display: none;
   max-width: $toast-max-width;
   overflow: hidden; // cheap rounded corners on nested items
   font-size: $toast-font-size; // knock it down to 14px
@@ -14,6 +15,10 @@
   }
 }
 
+.toast.show {
+  display: inherit;
+}
+
 .toast-header {
   display: flex;
   align-items: center;