(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

static/ui-icons/UI/Copy.svg

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg width="16px" height="16px" viewBox="0 0 16 16" version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
+    <!-- Generator: Sketch 63.1 (92452) - https://sketch.com -->
+    <title>Icons / UI / Copy</title>
+    <desc>Created with Sketch.</desc>
+    <g id="Icons-/-UI-/-Copy" stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
+        <path d="M11,3 L11,15 L2,15 L2,3 L11,3 Z M10,3.999 L3,3.999 L3,13.999 L10,13.999 L10,3.999 Z M14,0 L14,12 L11.5,12 L11.5,11 L13,11 L13,1 L6,1 L6,2.5 L5,2.5 L5,0 L14,0 Z" id="Combined-Shape" fill="#000000" fill-rule="nonzero"></path>
+    </g>
+</svg>