(core) complete light sign-up flow for appsumo, and customize summaries

Summary:
Current appsumo sign-up flow doesn't reach the billing pages.
This diff nudges user on through that extra step.

It also tweaks plan summaries to say what special appsumo
features are in effect (member count prepaid for).

Test Plan: manual

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2882

app/server/lib/FlexServer.ts

@@ -668,6 +668,24 @@ export class FlexServer implements GristServer {
           const prefix = isOrgInPathOnly(req.hostname) ? `/o/${mreq.org}` : '';
           return res.redirect(`${prefix}/welcome/user`);
         }
+        if (mreq.org && mreq.org.startsWith('o-')) {
+          // We are on a team site without a custom subdomain.
+          // If the user is a billing manager for the org, and the org
+          // is supposed to have a custom subdomain, forward the user
+          // to a page to set it.
+
+          // TODO: this is more or less a hack for AppSumo signup flow,
+          // and could be removed if/when signup flow is revamped.
+
+          // If "welcomeNewUser" is ever added to billing pages, we'd need
+          // to avoid a redirect loop.
+
+          const orgInfo = this.dbManager.unwrapQueryResult(await this.dbManager.getOrg({userId: user.id}, mreq.org));
+          if (orgInfo.billingAccount.isManager && orgInfo.billingAccount.product.features.vanityDomain) {
+          const prefix = isOrgInPathOnly(req.hostname) ? `/o/${mreq.org}` : '';
+          return res.redirect(`${prefix}/billing/payment?billingTask=signUpLite`);
+          }
+        }
         next();
       });