New crate crates/features (inbuxa-features), AGPL-3.0-only, holding the
tenancy rules. Hooks in common: a tenant's roles and permission lists cap
its people's permissions; a change to a tenant, or to a role a tenant holds,
drops its members' cached permissions; delivery and every other write check
the tenant's maxDiskQuota; usedDiskQuota reads the tenant usage counter.
The default Tenant Administrator role gains sysTenantGet and sysTenantQuery.