A set of tweaks to simplify electron packaging (#421)

* Replace `ormconfig.js` with a newer mechanism of configuring
    TypeORM that can be included in the source code properly.
    The path to `ormconfig.js` has always been awkward to handle,
    and eliminating the file makes building different Grist setups
    a bit simpler.
  * Remove `electron` package. It is barely used, just for some old
    remnants of an older attempt at electron packaging. It was used
    for two types, which I left at `any` for now. More code pruning is
    no doubt possible here, but I'd rather do it when Electron packaging
    has solidified.
  * Add a hook for replacing the login system, and for adding some
    extra middleware the login system may need.
  * Add support for some more possible locations of Python, which
    arise when a standalone version of it is included in the Electron
    package. This isn't very general purpose, just configurations
    that I found useful.
  * Support using grist-core within a yarn workspace - the only tweak
    needed was webpack related.
  * Allow an external ID to be optionally associated with documents.

app/client/lib/SafeBrowser.ts

@@ -40,7 +40,6 @@ import { getOriginUrl } from 'app/common/urlUtils';
 import { GristAPI, RPC_GRISTAPI_INTERFACE } from 'app/plugin/GristAPI';
 import { RenderOptions, RenderTarget } from 'app/plugin/RenderOptions';
 import { checkers } from 'app/plugin/TypeCheckers';
-import { IpcMessageEvent } from 'electron';
 import { IMsgCustom, IMsgRpcCall, Rpc } from 'grain-rpc';
 import { Disposable } from './dispose';
 const G = getBrowserGlobals('document', 'window');
@@ -316,7 +315,7 @@ class WebviewProcess extends ViewProcess {
     // TODO: find a way for keyboard events to play nice when webviews are non-modal.
     Mousetrap.setPaused(true);
     this.autoDisposeCallback(() => Mousetrap.setPaused(false));
-    webview.addEventListener('ipc-message', (event: IpcMessageEvent) => {
+    webview.addEventListener('ipc-message', (event: any /* IpcMessageEvent */) => {
       // The event object passed to the listener is missing proper documentation. In the examples
       // listed in https://electronjs.org/docs/api/ipc-main the arguments should be passed to the
       // listener after the event object, but this is not happening here. Only we know it is a