(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

stubs/app/client/ui/BillingButtons.ts

@@ -1,16 +0,0 @@
-import {AppModel} from 'app/client/models/AppModel';
-import {DomElementArg} from 'grainjs';
-
-export function buildUserMenuBillingItem(
-  _appModel: AppModel,
-  ..._args: DomElementArg[]
-) {
-  return null;
-}
-
-export function buildAppMenuBillingItem(
-  _appModel: AppModel,
-  ..._args: DomElementArg[]
-) {
-  return null;
-}

stubs/app/client/ui/WelcomeCoachingCall.ts

@@ -1 +0,0 @@
-export * from 'app/client/ui/WelcomeCoachingCallStub';