If we don't revoke sessions, this can lead to issues when we import a
realm (e.g. from Slack) into the original subdomain for the same
organization. Because then, users who already had sessions before the
move end up stuck:
- when they try to open Zulip on the original subdomain, which now has
the imported instance, they get redirected to the moved subdomain due to
their pre-existing session cookie.
- they can't even log out to get rid of the bugged cookie, because the
account they're logged into in that session is now on the moved
subdomain.
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