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/adminDirectory.ts

@@ -63,6 +63,8 @@ export interface DirectoryAccount {
 export interface DirectoryDomain {
   id: string;
   name: string;
+  /** The tenant the domain is in: an account can be in a tenant only on one of its domains. */
+  memberTenantId?: string | null;
 }
 
 const ACCOUNT_PROPERTIES = [
@@ -121,7 +123,7 @@ async function all<T>(object: "Domain" | "Role", properties: string[]): Promise<
   return res.list;
 }
 
-export const listDomains = () => all<DirectoryDomain>("Domain", ["name"]);
+export const listDomains = () => all<DirectoryDomain>("Domain", ["name", "memberTenantId"]);
 export const listRoles = () => all<RoleDef>("Role", ["description", "enabledPermissions", "roleIds"]);
 
 export async function listGroups(): Promise<DirectoryAccount[]> {