Administrators can easily switch between skins (without affecting their

Administrators can easily switch between skins (without affecting their
default skin), making tests with a specific skin much easier.

Note: Maybe it might be a better idea to enable this feature for all
users when the site is in debug mode and hide this (even for
administrators) when the site is in release mode.

Signed-off-by: Florent Bruneau <florent.bruneau@polytechnique.org>