(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

app/client/ui/TopBar.ts

@@ -26,7 +26,7 @@ const t = makeT('TopBar');
 export function createTopBarHome(appModel: AppModel) {
   return [
     cssFlexSpace(),
-
+    appModel.supportGristNudge.showButton(),
     (appModel.isTeamSite && roles.canEditAccess(appModel.currentOrg?.access || null) ?
       [
         basicButton(