Fix validation feedback icon in select multiple

Validation feedback for `<select multiple>` should look like
`<textarea>`.

The previous implementation was placing the validation icon in the
middle of the select field together with the single select arrow, that
is not supposed to be part of this kind of inputs

scss/mixins/_forms.scss

@@ -92,6 +92,14 @@
         background-image: escape-svg($form-select-indicator), escape-svg($icon);
         background-position: $form-select-bg-position, $form-select-feedback-icon-position;
         background-size: $form-select-bg-size, $form-select-feedback-icon-size;
+
+        &[multiple],
+        &[size]:not([size="1"]) {
+          padding-right: $input-height-inner;
+          background-image: escape-svg($icon);
+          background-position: top $input-height-inner-quarter right $input-height-inner-quarter;
+          background-size: $form-select-feedback-icon-size;
+        }
       }
 
       &:focus {