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
