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.
I find it a bit strange that we have a `TenantsTestUtil`, but still have things like `createTenantRestContext` in the general `TestUtil` file. Wouldn't it make more sense to move `generateTestTenantAlias` into the main `TestUtil`