Follow what the live server does with tenants and domains

A run on the production server with throwaway tenants, a role, lists and a
domain, all removed, found three things the source reading had not:

- Something in a tenant has to be on a domain in that tenant (a list in a
  tenant on an unassigned domain is invalidForeignKey), while something in
  no tenant may be on a tenant's domain. The account panel's tenant choice
  offered every tenant; it offers only the domain's now, and a new account
  starts in the tenant of the domain it is made on. The domain list reads
  memberTenantId for it.
- A domain created in a tenant puts its DKIM keys there too, and they keep
  the tenant from being deleted. They are counted with the rest, so Delete
  is not offered while any remain.
- Stalwart lets a domain leave a tenant while the tenant still has accounts
  on it, stranding them. The panel asks first and refuses while any are
  there.

The refusal to delete a tenant that holds anything was confirmed, as were
tenant create, quota pointers, logo and rename. The mock follows the domain
rule, filters DKIM keys by tenant, and KNOWN-ISSUES records the run.

The non-Enterprise notice is now just "Tenants are a Stalwart Enterprise
feature." Two sentences were reworded and one plural added, in all nine
catalogues, and the old sentences are gone.

web/src/lib/__tests__/adminTenants.test.ts

@@ -21,7 +21,7 @@ describe("what a tenant holds", () => {
       if (method === "x:Role/query") throw new Error("forbidden");
       return { total: method === "x:Account/query" && f["@type"] === "Group" ? 2 : 1 };
     });
-    expect(await countTenantMembers("t1")).toEqual({ accounts: 1, groups: 2, lists: 1, domains: 1 });
+    expect(await countTenantMembers("t1")).toEqual({ accounts: 1, groups: 2, lists: 1, domains: 1, dkimKeys: 1 });
     expect(call).toHaveBeenCalledWith("x:Account/query", { filter: { "@type": "User", memberTenantId: "t1" }, limit: 0, calculateTotal: true });
     expect(call).toHaveBeenCalledWith("x:Domain/query", { filter: { memberTenantId: "t1" }, limit: 0, calculateTotal: true });
     call.mockRestore();