User.from_uid method should validate that uid is valid

[maxkadel]

We want administrators to be able to create new users without them having previously logged in, but we also want to verify that the given uid is a valid NetID, and does not have a typo in it.

Implementation notes

It might make sense to connect to the ldap server to verify the UID. See https://github.com/pulibrary/approvals/blob/16fec05410196bc3be689784c4a9e919a110102a/app/services/ldap.rb for a possible example implementation.