Ensure tenant alias is lower-case

We should ensure that the tenant alias of a tenant is lower case when creating / updating tenants in the admin UI.

Not sure at this point if we should validate and reject mixed-case tenant aliases or if we should automatically lower-case them. Probably should just do it for the administrator.
`createTenantRestContext` creates a rest context, it doesn't really have anything to do with the tenants api ?

If something is specific to a particular api such as generating a valid unique tenant alias is to tenants, I think it's better in that module's test util.