Fix bug in ace-src-noconflict with Relative url.

If we doesn't use a relative url we have this:
<pre>gon.relative_url_root=""</pre>
Else <pre>gon.relative_url_root="/gitlab"</pre>

Gitlab::Application.config.assets.prefix contains:
<pre>/assets</pre>
Merge pull request #4826 from amouhzi/patch-1

Fix bug in ace-src-noconflict with Relative url.