Allow admin to specify domain for email auth links

Similar to https://github.com/alphagov/notifications-api/pull/1515

This lets the admin app pass in a domain to use for email auth links,
so that when it’s running on a different URL users who try to sign in
will get an email auth link for the domain they sign in on, not the
default admin domain for the environment in which the API is running.

app/user/users_schema.py

@@ -22,6 +22,7 @@ post_send_user_email_code_schema = {
         # doesn't need 'to' as we'll just grab user.email_address. but lets keep it
         # as allowed to keep admin code cleaner, but only as null to prevent confusion
         'to': {'type': 'null'},
+        'email_auth_link_host': {'type': ['string', 'null']},
         'next': {'type': ['string', 'null']},
     },
     'required': [],