Note that if there was a non-lowercase user already in the DB, it cannot
be found again. To fix this, run in the DB:
sqlite> UPDATE users SET user_id = LOWER(user_id);
They can always be empty. This simplifies mutation, since graphql_client
doesn't have an easy way to conditionally leave out fields (we could do
that with `@include`, but that's one bool per field in addition to the
field, a bit ugly).
