[Issue 91]: Optionally use netid as user unique identifier

[wwelling]

Migration script to update weaver_users with duplicate netid after migrating uin to netid:

WITH duplicate_netids AS (
    SELECT netid, COUNT(*) AS count FROM weaver_users GROUP BY netid HAVING COUNT(*) > 1
)
UPDATE weaver_users SET netid = NULL FROM duplicate_netids WHERE weaver_users.netid = duplicate_netids.netid AND weaver_users.email NOT LIKE '%tamu.edu';

Migration script to add unique constraint on weaver_users netid:

ALTER TABLE weaver_users ADD CONSTRAINT weaver_users_netid UNIQUE (netid);