(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/Clipboard.css

@@ -0,0 +1,13 @@
+/**
+ * With some guidance from Lucidchart:
+ * https://www.lucidchart.com/techblog/2014/12/02/definitive-guide-copying-pasting-javascript/
+ */
+textarea.copypaste {
+  position: absolute;
+  top: -100px;
+  left: 0;
+  width: 10px;
+  height: 10px;
+  font-size: 1;
+  z-index: -1;
+}