We're migrating to using the cleaner zulip.com domain, which involves
changing all of our links from ReadTheDocs and other places to point
to the cleaner URL.
This provides a nice user experience for folks where we do know what
their LDAP credentials are.
Though we need to fix#10917 before the content in the email with be
correct.
I think an hour after signup is not the right time to try to get someone to
re-engage with a product.
This also makes the day1 email clearly a transactional email both in
experiencing the product and in the eyes of various anti-spam laws, and
allows us to remove the unsubscribe link.