(core) Add Support Grist page and nudge

Summary:
Adds a new Support Grist page (accessible only in grist-core), containing
options to opt in to telemetry and sponsor Grist Labs on GitHub.

A nudge is also shown in the doc menu, which can be collapsed or permanently
dismissed.

Test Plan: Browser and server tests.

Reviewers: paulfitz, dsagal

Reviewed By: paulfitz

Subscribers: jarek, dsagal

Differential Revision: https://phab.getgrist.com/D3926

buildtools/webpack.config.js

@@ -13,9 +13,7 @@ module.exports = {
   entry: {
     main: "app/client/app",
     errorPages: "app/client/errorMain",
-    account: "app/client/accountMain",
     billing: "app/client/billingMain",
-    activation: "app/client/activationMain",
     // Include client test harness if it is present (it won't be in
     // docker image).
     ...(fs.existsSync("test/client-harness/client.js") ? {