[FIX] website: exception handling: let redirects pass untouched

The website module normally tries to render user-friendly
error pages for any error occurring during the processing
of a website-enabled request.
This happens even for werkzeug's HTTPExceptions that have
their own response, because we want the website layout to
be applied on top of the error page.

One special sort of HTTPException should be preserved
without rewriting: the manually-crafted ones generated
with abort(), and usually wrapping a redirect response,
which bears no alteration.
[FIX] website: avoid hiding the signin info for users who are not employees nor portal users

This happens when you do a plain signup, you
end up as neither an employee nor a portal user,
but should still now when you are logged in.