Merge branch 'highlight-link-sidebar' into 'master'

prettier highlighting of active link in the sidebar

Before:

![Screenshot_2015-06-25_11.17.08](https://dev.gitlab.org/gitlab/gitlabhq/uploads/851606a122c1d1174d0f97857ad472d3/Screenshot_2015-06-25_11.17.08.png)

After:

![Screenshot_2015-06-25_11.17.42](https://dev.gitlab.org/gitlab/gitlabhq/uploads/b8dc55bcd76dac29652ef5d25f99ccff/Screenshot_2015-06-25_11.17.42.png)

Only with this theme it's not an improvement (this is an honest merge request (trademark pending)) :

![Screenshot_2015-06-25_11.18.35](https://dev.gitlab.org/gitlab/gitlabhq/uploads/5d631a11ee866f5fc51e985b02a836ba/Screenshot_2015-06-25_11.18.35.png)

See merge request !1879

app/assets/stylesheets/themes/gitlab-theme.scss

@@ -72,7 +72,7 @@
 
       &.active a {
         color: #FFF;
-        font-weight: bold;
+        background: $color-dark;
 
         &.no-highlight {
           border: none;