Fix theme editor when using apache redirection

When using a simple apache configuration, theme editor is not behaving correctly, producing a "page not found" error in a javascript popup when trying to edit a fragment (for instance).

Error is not visible when loading the theme editor, it becomes apparent when trying to edit the theme.

Steps to reproduce:

• install apache2
• setup proxy and proxy_http modes (a2enmod proxy, a2enmod proxy_http)
• setup redirection:

<VirtualHost *:80>

ProxyVia On
ProxyRequests Off
#RewriteEngine On

ProxyPass /nuxeo/ http://127.0.0.1:8080/nuxeo/
ProxyPassReverse /nuxeo/ http://127.0.0.1:8080/nuxeo/

</VirtualHost>

Looking at the html source, the base path for theme is wrong:
var nxthemesBasePath = "/nuxeo";
instead of
var nxthemesBasePath = "/nuxeo/site";