(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/Fields/FieldFunction.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 / Fields / FieldFunction</title>
+    <desc>Created with Sketch.</desc>
+    <g id="Icons-/-Fields-/-FieldFunction" stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
+        <path d="M3.05903249,12.373047 L4.10849891,6.46979836 L3,6.46979836 L3.16397913,5.49248276 L4.28559637,5.49248276 L4.33806969,5.13828784 C4.52828548,3.69527151 5.32850363,3 6.79119746,3 C7.05356406,3 7.34872649,3.02623666 7.59141561,3.07215082 L7.41431815,4.01011143 C7.30281234,3.98387477 7.02076824,3.96419728 6.81743412,3.96419728 C6.05657096,3.96419728 5.68269855,4.29215553 5.53183775,5.08581452 L5.45968693,5.49248276 L7.04044573,5.49248276 L6.86334827,6.46979836 L5.28914864,6.46979836 L4.25280054,12.373047 L3.05903249,12.373047 Z M10.9934359,12.373047 L9.72751706,9.76249927 L9.6816029,9.76249927 L7.63514337,12.373047 L6.29051452,12.373047 L9.11095553,8.89668947 L7.37933593,5.47280526 L8.6846098,5.47280526 L9.93085118,8.12270798 L9.98988366,8.12270798 L12.0494615,5.47280526 L13.420327,5.47280526 L10.5342944,8.94260363 L12.305269,12.373047 L10.9934359,12.373047 Z" id="fx" fill="#000000" fill-rule="nonzero"></path>
+    </g>
+</svg>