(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/FieldInteger.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 / FieldInteger</title>
+    <desc>Created with Sketch.</desc>
+    <g id="Icons-/-Fields-/-FieldInteger" stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
+        <path d="M13,13 L13,3 L12,3 L12,2 L14,2 L14,3 L14,13 L14,14 L12,14 L12,13 L13,13 Z M2,13 L2,3 L2,2 L4,2 L4,3 L3,3 L3,13 L4,13 L4,14 L2,14 L2,13 Z M8.90234375,12 L7.8359375,12 L7.8359375,5.17382812 L7.78320312,5.17382812 L5.93164062,6.50976562 L5.93164062,5.40820312 L7.8359375,4.00195312 L8.90234375,4.00195312 L8.90234375,12 Z" id="Combined-Shape" fill="#000000" fill-rule="nonzero"></path>
+    </g>
+</svg>