Add z-index to `is-loading` class (#1497)

The `has-addons` class for a field can hide the loader of its children
if they have the `is-loading` class when the field is hovered or active.

sass/elements/form.sass

@@ -611,6 +611,7 @@ $help-size: $size-small !default
       position: absolute !important
       right: 0.625em
       top: 0.625em
+      z-index: 4
     &.is-small:after
       font-size: $size-small
     &.is-medium:after