(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/SafeBrowserProcess.css

@@ -0,0 +1,12 @@
+.plugin_instance_fullscreen {
+  position: absolute;
+  width: 100%;
+  height: 100%;
+  left: 0;
+  top: 0;
+  z-index:9999;
+}
+
+.safe_browser_process{
+  border: none;
+}