Display asc/desc sort indicator from icon font

Signed-off-by: Alex Coles <alex@alexbcoles.com>

app/assets/stylesheets/content/_links.sass

@@ -43,11 +43,20 @@ a
     padding-right: 16px
     background-position: 100% 50%
     background-repeat: no-repeat
-    &.asc
-      background-image: url(image-path('sort_asc.png'))
+    &.asc:after
+      @include icon-common
+      content: "\e0dd"
+      margin-left: 5px
+      font-size: 1.2em
+      vertical-align: text-bottom
+
+    &.desc:after
+      @include icon-common
+      content: "\e0de"
+      margin-left: 5px
+      font-size: 1.2em
+      vertical-align: text-bottom
 
-    &.desc
-      background-image: url(image-path('sort_desc.png'))
   &.issue.closed, &.issue.closed:hover
     text-decoration: line-through