(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/common/Install.ts

@@ -0,0 +1,10 @@
+import {TelemetryLevel} from 'app/common/Telemetry';
+
+export interface InstallPrefs {
+  telemetry?: TelemetryPrefs;
+}
+
+export interface TelemetryPrefs {
+  /** Defaults to "off". */
+  telemetryLevel?: TelemetryLevel;
+}