For users who are not logged in and for those who don't have
'prefers_web_public_view' set in session, we redirect them
to the default login page where they can choose to login
as spectator or authenticated user.
This reverses the policy that was set, but incompletely enforced, by
commit 951514dd7d. The self-closing tag
syntax is clearer, more consistent, simpler to parse, compatible with
XML, preferred by Prettier, and (most importantly now) required by
FormatJS.
Signed-off-by: Anders Kaseorg <anders@zulip.com>