Add counter of selected templates and folders

Being able to see how many things you have selected gives you positive
feedback that reinforces that what you’ve done has been recognised. It
helps you understand the implications of your actions (ie you see ‘3
selected’ before you press the ‘Move’ button). And it gives you an
escape hatch the get out of the state you’re in by providing the ‘Clear’
button.

We also found in prototyping that having a ‘Nothing selected’ message
helps draws people’s attention to the checkboxes when they first
encounter the folders feature.

This commit implements the counter and the cancel button. It tries to
follow the existing patterns for this module.

app/assets/stylesheets/components/message.scss

@@ -108,6 +108,13 @@
     padding: $gutter-half 0 $gutter-one-third 0;
   }
 
+  &-selected-counter {
+    position: absolute;
+    right: $gutter-half;
+    top: $gutter - 1px;
+    color: $secondary-text-colour;
+  }
+
 }
 
 .folder-heading {