(core) Add optional telemetry to grist-core

Summary:
Adds support for optional telemetry to grist-core.

A new environment variable, GRIST_TELEMETRY_LEVEL, controls the level of telemetry collected.

Test Plan: Server and unit tests.

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: dsagal, anaisconce

Differential Revision: https://phab.getgrist.com/D3880

app/client/models/errors.ts

@@ -215,6 +215,6 @@ export function logError(error: Error|string) {
   }).catch(e => {
     // There ... isn't much we can do about this.
     // tslint:disable-next-line:no-console
-    console.warn('Failed to log event', event);
+    console.warn('Failed to log event', e);
   });
 }