freshen app/client/ui2018/cssVars.ts

The cssVars.ts file has changed to include some more knobs
for custom theming.  This commit updates the file, and
introduces a `stubs` directory for stubbing code that is
specific to our deployments of Grist and not of general interest.

buildtools/tsconfig-base.json

@@ -16,6 +16,7 @@
       "*": [
         "*",
         "grist-core/*",
+        "stubs/*"
       ],
     },
     "composite": true,

buildtools/webpack.config.js

@@ -34,6 +34,7 @@ module.exports = {
   resolve: {
     modules: [
       path.resolve('./_build'),
+      path.resolve('./_build/stubs'),
       path.resolve('./node_modules')
     ],
   },