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/gen-server/entity/Document.ts

@@ -99,6 +99,9 @@ export class Document extends Resource {
         if (props.options.icon !== undefined) {
           this.options.icon = sanitizeIcon(props.options.icon);
         }
+        if (props.options.externalId !== undefined) {
+          this.options.externalId = props.options.externalId;
+        }
         // Normalize so that null equates with absence.
         for (const key of Object.keys(this.options) as Array<keyof DocumentOptions>) {
           if (this.options[key] === null) {