(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/components/ViewPane.ts

@@ -0,0 +1,6 @@
+// This module is unused except to group some modules for a webpack bundle.
+// TODO It is a vestige of the old ViewPane.js, and can go away with some bundling improvements.
+
+import * as FieldConfigTab from 'app/client/components/FieldConfigTab';
+import * as ViewConfigTab from 'app/client/components/ViewConfigTab';
+export {FieldConfigTab, ViewConfigTab};