(core) fix browser check and favicon in grist-core

Summary:
A check for old browsers and a Grist favicon were not available in
grist-core, leaving harmless but distracting errors in logs. This
adds them.

Test Plan: checked manually

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: jarek

Differential Revision: https://phab.getgrist.com/D3207

buildtools/webpack.check.js

@@ -0,0 +1,11 @@
+const path = require('path');
+
+module.exports = {
+  target: 'web',
+  mode: 'production',
+  entry: "./_build/app/client/browserCheck.js",
+  output: {
+    path: path.resolve("./static"),
+    filename: "browser-check.js"
+  },
+};