Increase specificity of simple-search icon

We were seeing the styles being over-ridden if icomoon.css was injected
after simple-search.scss.

Fixes #1764

h/static/styles/simple-search.scss

@@ -16,7 +16,8 @@
   box-sizing: border-box;
 }
 
-.simple-search-icon {
+@include icons {
+  &.simple-search-icon {
     float: left;
     display: inline;
     font-size: 1.1em;
@@ -24,10 +25,11 @@
     margin-left: -100%;
     position: relative;
     right: 19 / 14 * 1em;
+  }
+}
 
-  :not(:focus) ~ & {
+:not(:focus) ~ .simple-search-icon {
   color: $gray-lighter;
-  }
 }
 
 .simple-search-input {