This adds a web flow and management command for reactivating a Zulip
organization, with confirmation from one of the organization
administrators.
Further work is needed to make the emails nicer (ideally, we'd send
one email with all the admins on the `To` line, but the `send_email`
library doesn't support that).
Fixes#10783.
With significant tweaks to the email text by tabbott.
Raymond Akornor