(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/Tick.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 52.5 (67469) - http://www.bohemiancoding.com/sketch -->
+    <title>Icons / UI / Tick</title>
+    <desc>Created with Sketch.</desc>
+    <g id="Icons-/-UI-/-Tick" stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
+        <path d="M11.6183069,4.67702847 C11.7966789,4.46622517 12.1121678,4.43993443 12.3229711,4.61830645 C12.5337744,4.79667848 12.5600652,5.11216741 12.3816931,5.32297071 L6.53068827,12.2377946 L3.64644661,9.35355298 C3.45118446,9.15829084 3.45118446,8.84170835 3.64644661,8.6464462 C3.84170876,8.45118406 4.15829124,8.45118406 4.35355339,8.6464462 L6.46931173,10.7622045 L11.6183069,4.67702847 Z" id="Path-4" fill="#000000" fill-rule="nonzero"></path>
+    </g>
+</svg>