(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/hashingUtils.ts

@@ -0,0 +1,12 @@
+import {createHash} from 'crypto';
+
+/**
+ * Returns a hash of `id` prefixed with the first 4 characters of `id`. The first 4
+ * characters are included to assist with troubleshooting.
+ *
+ * Useful for situations where potentially sensitive identifiers are logged, such as
+ * doc ids of docs that have public link sharing enabled.
+ */
+export function hashId(id: string): string {
+  return `${id.slice(0, 4)}:${createHash('sha256').update(id.slice(4)).digest('base64')}`;
+}