(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/gen-server/lib/Activations.ts

@@ -20,6 +20,7 @@ export class Activations {
       if (!activation) {
         activation = manager.create(Activation);
         activation.id = makeId();
+        activation.prefs = {};
         await activation.save();
       }
       return activation;