Say tenants are Enterprise only where the installation asks, as the demo will

On a server that is not Enterprise the Tenants page is still only the
notice. On Enterprise the notice is gone -- a real installation that has
tenants has the licence -- unless SHOW_ENTERPRISE_NOTICES=1 asks for it above
the list. The public demo will set it: it reports Enterprise so tenants can be
shown, and should not suggest they come without the licence. The setting
reaches the browser as session.ihasmail.server.enterpriseNotices.

server/src/app.ts

@@ -896,7 +896,12 @@ function sessionExtras(session: LiveSession, info: AccountInfo = { locale: null,
        * session that may administer -- where the operator says its own
        * administration is.
        */
-      server: { edition: info.edition, adminUrl: administrationAllowed(config.administration, session.remember) ? adminUrlFor(session.username) : null },
+      server: {
+        edition: info.edition,
+        adminUrl: administrationAllowed(config.administration, session.remember) ? adminUrlFor(session.username) : null,
+        /** SHOW_ENTERPRISE_NOTICES: say "Enterprise feature" on Enterprise too, as the demo does. */
+        enterpriseNotices: config.showEnterpriseNotices,
+      },
       /**
        * Whether this session may administer: the installation offers it
        * (ADMINISTRATION) and the person signed in on a device marked as their own.