Removed tenant_name from User model.
Added new model Tenant - to keep all tenants in one place.
Added new model Role - stores user roles.
Added new model UserRight - join table for user, tenant and role.

Each user can have multiple roles for multiple tenants.
E.G. User John Doe can be an administrator in a.domain.com and a moderator
in b.domain.com and a user at c.domain.com