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/common/UserAPI.ts

@@ -115,6 +115,8 @@ export interface DocumentOptions {
   description?: string|null;
   icon?: string|null;
   openMode?: OpenDocMode|null;
+  externalId?: string|null;  // A slot for storing an externally maintained id.
+                             // Not used in grist-core, but handy for Electron app.
 }
 
 export interface DocumentProperties extends CommonProperties {