(core) Add basic activation page to grist-ee

Summary:
Adds an activation page to grist-ee that currently shows activation status.

Follow-up diffs will introduce additional functionality, such as the ability to
enter activation keys directly from the activation page.

Test Plan: No grist-ee tests (yet).

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: dsagal, paulfitz

Differential Revision: https://phab.getgrist.com/D3582

buildtools/webpack.config.js

@@ -8,6 +8,8 @@ module.exports = {
     main: "app/client/app",
     errorPages: "app/client/errorMain",
     account: "app/client/accountMain",
+    billing: "app/client/billingMain",
+    activation: "app/client/activationMain",
   },
   output: {
     filename: "[name].bundle.js",