(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/models/TelemetryModel.ts

@@ -0,0 +1,32 @@
+import {AppModel, getHomeUrl} from 'app/client/models/AppModel';
+import {TelemetryPrefs} from 'app/common/Install';
+import {InstallAPI, InstallAPIImpl, TelemetryPrefsWithSources} from 'app/common/InstallAPI';
+import {bundleChanges, Disposable, Observable} from 'grainjs';
+
+export interface TelemetryModel {
+  /** Telemetry preferences (e.g. the current telemetry level). */
+  readonly prefs: Observable<TelemetryPrefsWithSources | null>;
+  fetchTelemetryPrefs(): Promise<void>;
+  updateTelemetryPrefs(prefs: Partial<TelemetryPrefs>): Promise<void>;
+}
+
+export class TelemetryModelImpl extends Disposable implements TelemetryModel {
+  public readonly prefs: Observable<TelemetryPrefsWithSources | null> = Observable.create(this, null);
+  private readonly _installAPI: InstallAPI = new InstallAPIImpl(getHomeUrl());
+
+  constructor(_appModel: AppModel) {
+    super();
+  }
+
+  public async fetchTelemetryPrefs(): Promise<void> {
+    const prefs = await this._installAPI.getInstallPrefs();
+    bundleChanges(() => {
+      this.prefs.set(prefs.telemetry);
+    });
+  }
+
+  public async updateTelemetryPrefs(prefs: Partial<TelemetryPrefs>): Promise<void> {
+    await this._installAPI.updateInstallPrefs({telemetry: prefs});
+    await this.fetchTelemetryPrefs();
+  }
+}