(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/CrossBig.svg

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg width="24px" height="24px" viewBox="0 0 24 24" version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
+    <!-- Generator: Sketch 52.5 (67469) - http://www.bohemiancoding.com/sketch -->
+    <title>Icons / UI / CrossBig</title>
+    <desc>Created with Sketch.</desc>
+    <g id="Icons-/-UI-/-CrossBig" stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
+        <path d="M13.4142136,12 L19.7071068,18.2928932 C20.0976311,18.6834175 20.0976311,19.3165825 19.7071068,19.7071068 C19.3165825,20.0976311 18.6834175,20.0976311 18.2928932,19.7071068 L12,13.4142136 L5.70710678,19.7071068 C5.31658249,20.0976311 4.68341751,20.0976311 4.29289322,19.7071068 C3.90236893,19.3165825 3.90236893,18.6834175 4.29289322,18.2928932 L10.5857864,12 L4.29289322,5.70710678 C3.90236893,5.31658249 3.90236893,4.68341751 4.29289322,4.29289322 C4.68341751,3.90236893 5.31658249,3.90236893 5.70710678,4.29289322 L12,10.5857864 L18.2928932,4.29289322 C18.6834175,3.90236893 19.3165825,3.90236893 19.7071068,4.29289322 C20.0976311,4.68341751 20.0976311,5.31658249 19.7071068,5.70710678 L13.4142136,12 Z" id="Combined-Shape" fill="#000000" fill-rule="nonzero"></path>
+    </g>
+</svg>