Include Tenant in the database model

Create a TenantEntity to model tenants and include a FK to tenants in UserEntity and we'll also need this for MetadataSchemaEntity as well. MetadataSchemaEntity.name will need to be unique for a given tenant_id. See https://gist.github.com/machristie/0c7e28f11347d735f5517c3b5bf14d57 for schema design.

Start by locating TenantEntity, UserEntity, and MetadataSchemaEntity in the Java model and review the linked schema design. Trace the existing entity relationships and uniqueness constraints before deciding how the tenant foreign keys fit. Done means tenants are modeled, both entities reference them, and MetadataSchemaEntity.name is unique within a tenant.

Written by the indexing model from the issue text.