Fix and improves the 'become user' feature.

Fix and improves the 'become user' feature.

You can now select an account type among all available types and see the
site with the permissions of this type. Leaving a SUID session go back to
the page where the SUID session has been started.

Note: The site seen by someone with this account type might be a bit
different since some parameters are independent from the type (like the
fact your account is attached to a profile).

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