Use a Font Awesome icon in locale switcher

We've deprecated the "icons" font since we started using Font Awesome
two years ago.

app/assets/stylesheets/layout/locale_switcher.scss

@@ -1,18 +1,20 @@
 .locale {
+  @include has-fa-icon(angle-down, solid, after);
   float: left;
-  height: $line-height * 1.5;
+  margin-bottom: $line-height / 4;
   margin-left: $line-height / 2;
+  margin-top: $line-height / 4;
   position: relative;
 
   &::after {
     color: #808080;
-    content: "\61";
-    font-family: "icons" !important;
     font-size: $small-font-size;
+    margin-right: 0;
     pointer-events: none;
     position: absolute;
     right: 2px;
-    top: 9px;
+    top: 50%;
+    transform: translateY(-50%);
   }
 
   .locale-form {
@@ -33,7 +35,6 @@
       font-size: $small-font-size;
       height: $line-height;
       margin-bottom: 0;
-      margin-top: $line-height / 4;
       outline: none;
       padding-left: $line-height / 4;
       padding-right: calc(#{$line-height / 4} + 1em);