auth: Add an organization reactivation flow with admin confirmation.

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.

zproject/urls.py

@@ -458,6 +458,10 @@ i18n_urls = [
     url(r'^new/(?P<creation_key>[\w]+)$',
         zerver.views.registration.create_realm, name='zerver.views.create_realm'),
 
+    # Realm Reactivation
+    url(r'^reactivate/(?P<confirmation_key>[\w]+)', zerver.views.realm.realm_reactivation,
+        name='zerver.views.realm.realm_reactivation'),
+
     # Global public streams (Zulip's way of doing archives)
     url(r'^archive/streams/(?P<stream_id>\d+)/topics/(?P<topic_name>[^/]+)$',
         zerver.views.archive.archive,