Simplify icon style selectors

All FontAwesome icons have an `fa` class, so just use that.

app/assets/stylesheets/base/gl_bootstrap.scss

@@ -117,7 +117,7 @@
         color: #888;
         text-shadow: 0 1px 1px #fff;
       }
-      i[class~="fa"] {
+      i.fa {
         line-height: 14px;
       }
     }

app/assets/stylesheets/pages/notes.scss

@@ -136,7 +136,7 @@ ul.notes {
     display: none;
     float: right;
 
-    [class~="fa"] {
+    i.fa {
       font-size: 16px;
       line-height: 16px;
       vertical-align: middle;