(core) Add product for new personal plan

Summary:
Adds the new personal plan as a product that will be available
in the future. Can be enabled along with other plan-related via
an environment variable.

Test Plan: Browser tests and existing tests.

Reviewers: jarek

Reviewed By: jarek

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D3533

app/common/gristUrls.ts

@@ -539,6 +539,9 @@ export interface GristLoadConfig {
 
   // String to append to the end of the HTML document.title
   pageTitleSuffix?: string;
+
+  // TODO: can be removed once new deal is released.
+  newDeal?: boolean;
 }
 
 export const HideableUiElements = StringUnion("helpCenter", "billing", "templates", "multiSite", "multiAccounts");