(core) Configure more comprehensive eslint rules for Typescript

Summary:
- Update rules to be more like we've had with tslint
- Switch tsserver plugin to eslint (tsserver makes for a much faster way to lint in editors)
- Apply suggested auto-fixes
- Fix all lint errors and warnings in core/, app/, test/

Test Plan: Some behavior may change subtly (e.g. added missing awaits), relying on existing tests to catch problems.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2785

app/server/lib/AppEndpoint.ts

@@ -4,7 +4,7 @@
  * of the client-side code.
  */
 import * as express from 'express';
-import fetch, {RequestInit, Response as FetchResponse} from 'node-fetch';
+import fetch, {Response as FetchResponse, RequestInit} from 'node-fetch';
 
 import {ApiError} from 'app/common/ApiError';
 import {getSlugIfNeeded, parseSubdomainStrictly} from 'app/common/gristUrls';