auth: Fix Apache SSO port number in confirmation links.

We had a report in the thread around
https://chat.zulip.org/#narrow/stream/31-production-help/topic/Apache-based.20SSO/near/741013
that confirmation links were taking the user to the /register form on
the Apache server, which of course doesn't work because the Apache
server architecture we have is intended to only serve a single
endpoint, /accounts/login/sso, and not any static assets (etc.).

This manifested as users getting a broke page with a bunch of JS
errors about missing static assets when trying to sign up for an
account.  The right fix is to ensure that we serve these confirmation
links (and maybe in the future, redirects) to the nginx server.

zerver/views/auth.py

@@ -125,7 +125,19 @@ def maybe_send_to_registration(request: HttpRequest, email: str, full_name: str=
             prereg_user.invited_as = invited_as
             prereg_user.save()
 
-        confirmation_link = create_confirmation_link(prereg_user, request.get_host(),
+        # We want to create a confirmation link to create an account
+        # in the current realm, i.e. one with a hostname of
+        # realm.host.  For the Apache REMOTE_USER_SSO auth code path,
+        # this is preferable over realm.get_host() because the latter
+        # contains the port number of the Apache instance and we want
+        # to send the user back to nginx.  But if we're in the realm
+        # creation code path, there might not be a realm yet, so we
+        # have to use request.get_host().
+        if realm is not None:
+            host = realm.host
+        else:
+            host = request.get_host()
+        confirmation_link = create_confirmation_link(prereg_user, host,
                                                      Confirmation.USER_REGISTRATION)
         if is_signup:
             return redirect(confirmation_link)