Refactor the tenant child model functionality for sub-tenancies

[ollieread]

The Eloquent child model functionality will need looking at, as it currently relies on the tenancy name coming from a method, and restricts usage of TenantRelation to a single method. I believe the following is the best way to improve this to better support sub-tenancies:

• Remove the reliance on the IsTenantChild::getTenancyName() method.
• Remove the restriction on the number of TenantRelation occurrences.
• Add a tenancy name option to TenantRelation, for models that can belong to multiple kinds of tenancies.