(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/server/lib/GristServer.ts

@@ -138,8 +138,11 @@ export function createDummyGristServer(): GristServer {
 
 export function createDummyTelemetry(): ITelemetry {
   return {
-    logEvent() { return Promise.resolve(); },
     addEndpoints() { /* do nothing */ },
-    getTelemetryLevel() { return 'off'; },
+    addPages() { /* do nothing */ },
+    start() { return Promise.resolve(); },
+    logEvent() { return Promise.resolve(); },
+    getTelemetryConfig() { return undefined; },
+    fetchTelemetryPrefs() { return Promise.resolve(); },
   };
 }