Add a DAO for User objects and fix up the User DB object (#1765)

First, we create a sequence of User IDs in Postgres and assign it to the
User ID field, meaning that Hibernate can autogenerate IDs.

Next, add an update timestamp.

Next, add a constraint that we can't have multiple Users with the same
email address.

Finally, create a DAO since we'll usually want to query by that email
address (at least for now).

db/src/main/resources/sql/schema/db-schema.sql.generated

@@ -742,7 +742,8 @@
     );
 
     create table "User" (
-       id int8 not null,
+       id  bigserial not null,
+        update_timestamp timestamptz,
         email_address text not null,
         gaia_id text not null,
         registry_lock_password_hash text,