(core) Makes EE frontend behave as core if EE isn't activated

Summary:
- Makes EE decide which ActivationPage to use
- Makes ProductUpgrades use core implementation if not activated
- Changes banners to proxy to core implementation if EE not activated
- [Fix] Enables new site creation in EE as in Core:
    - Core enables people to freely create new team sites.
    - Enterprise currently redirects to the pricing page.
    - This enables enterprise to also create team sites, instead of
    redirecting.

Test Plan: Manually test in EE, unit tests in Jenkins

Reviewers: paulfitz, jordigh

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D4264

stubs/app/client/components/Banners.ts

@@ -1,10 +1,2 @@
-import {AppModel} from 'app/client/models/AppModel';
-import {DocPageModel} from 'app/client/models/DocPageModel';
+export { buildHomeBanners, buildDocumentBanners } from 'app/client/components/CoreBanners';
 
-export function buildHomeBanners(_app: AppModel) {
-  return null;
-}
-
-export function buildDocumentBanners(_docPageModel: DocPageModel) {
-  return null;
-}

stubs/app/client/ui/ActivationPage.ts

@@ -1,12 +1,7 @@
-import {AppModel} from 'app/client/models/AppModel';
-import {Disposable} from 'grainjs';
+import {
+  DefaultActivationPage, IActivationPageCreator
+} from "app/client/ui/DefaultActivationPage";
 
-export class ActivationPage extends Disposable {
-  constructor(_appModel: AppModel) {
-    super();
-  }
-
-  public buildDom() {
-    return null;
-  }
+export function getActivationPage(): IActivationPageCreator {
+  return DefaultActivationPage;
 }