Before this commit, url preview of a theme was theme_xx/static/yyy.png
Now we make this url absolue /theme_xx/static/yyy.png
It was the expected behavior, and since werkeug 15.0 utils.redirect() don't use
'/' anymore as root but the current path.
Without this fix, theme selector uses /web/image/<id>, and the location become
/web/image/theme_xx/static/yyy.png instead of /theme_xx/static/yyy.png
closesodoo/odoo#51018
Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>