Allow null GAIA IDs for User objects (#1933)

We were under the mistaken impression before that there was a reliable
way to, out-of-band, get a GAIA ID for a particular email address.
Unfortunately, that isn't the case (at least, not in a scalable way or
one that support agents could use). As a result, we have to allow null
GAIA IDs in the database.

When we (or the support team) create new users, we will only specify the
email address and not the GAIA ID. Then, when the user logs in for the
first time, we will have the GAIA ID from the provided ID token, and we
can populate it then.

db/src/main/resources/sql/schema/nomulus.golden.sql

@@ -1085,7 +1085,7 @@ CREATE TABLE public."TmchCrl" (
 CREATE TABLE public."User" (
     id bigint NOT NULL,
     email_address text NOT NULL,
-    gaia_id text NOT NULL,
+    gaia_id text,
     registry_lock_password_hash text,
     registry_lock_password_salt text,
     global_role text NOT NULL,