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.

buildtools/webpack.api.config.js

@@ -1,5 +1,10 @@
 const path = require('path');
 
+// Get path to top-level node_modules if in a yarn workspace.
+// Otherwise node_modules one level up won't get resolved.
+// This is used in Electron packaging.
+const base = path.dirname(path.dirname(require.resolve('grainjs/package.json')));
+
 module.exports = {
   target: 'web',
   entry: {
@@ -18,7 +23,8 @@ module.exports = {
       path.resolve('.'),
       path.resolve('./ext'),
       path.resolve('./stubs'),
-      path.resolve('./node_modules')
+      path.resolve('./node_modules'),
+      base,
     ],
     fallback: {
       'path': require.resolve("path-browserify"),