(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/guessTimezone.ts

@@ -0,0 +1,11 @@
+import {loadMomentTimezone} from 'app/client/lib/imports';
+
+/**
+ * Returns the browser timezone, using moment.tz.guess(), allowing overriding it via a "timezone"
+ * URL parameter, for the sake of tests.
+ */
+export async function guessTimezone() {
+  const moment = await loadMomentTimezone();
+  const searchParams = new URLSearchParams(window.location.search);
+  return searchParams.get('timezone') || moment.tz.guess();
+}