landing_nav: Add tabnabbing protection to external target="_blank" link.
landing_nav: Add tabnabbing protection to external target="_blank" link.

This has no impact because zulip.com is not attacker-controlled, but
we should be consistent in protecting external target="_blank" links.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

zulip/zulip,andersk/zulip,zulip/zulip,zulip/zulip,andersk/zulip,rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip,zulip/zulip,zulip/zulip,rht/zulip,andersk/zulip,rht/zulip,rht/zulip,rht/zulip,andersk/zulip