(core) move client code to core

Summary:
This moves all client code to core, and makes minimal fix-ups to
get grist and grist-core to compile correctly.  The client works
in core, but I'm leaving clean-up around the build and bundles to
follow-up.

Test Plan: existing tests pass; server-dev bundle looks sane

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2627

app/client/lib/loadScript.js

@@ -0,0 +1,19 @@
+const Promise = require('bluebird');
+const G = require('./browserGlobals').get('document');
+
+/**
+ * Load dynamically an external JS script from the given URL. Returns a promise that is
+ * resolved when the script is loaded.
+ */
+function loadScript(url) {
+  return new Promise((resolve, reject) => {
+    let script = G.document.createElement("script");
+    script.type = "text/javascript";
+    script.onload = resolve;
+    script.onerror = reject;
+    script.src = url;
+    G.document.getElementsByTagName("head")[0].appendChild(script);
+  });
+}
+
+module.exports = loadScript;