(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/FieldSpinner.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 / FieldSpinner</title>
+    <desc>Created with Sketch.</desc>
+    <g id="Icons-/-Fields-/-FieldSpinner" stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
+        <path d="M11.1746043,10.1203717 C11.3842672,9.94066062 11.6999172,9.96494139 11.8796283,10.1746043 C12.0593394,10.3842672 12.0350586,10.6999172 11.8253957,10.8796283 L8.32539569,13.8796283 C8.13815082,14.0401239 7.86184918,14.0401239 7.67460431,13.8796283 L4.17460431,10.8796283 C3.96494139,10.6999172 3.94066062,10.3842672 4.1203717,10.1746043 C4.30008277,9.96494139 4.61573277,9.94066062 4.82539569,10.1203717 L8,12.8414611 L11.1746043,10.1203717 Z M4.82539569,5.8796283 C4.61573277,6.05933938 4.30008277,6.03505861 4.1203717,5.82539569 C3.94066062,5.61573277 3.96494139,5.30008277 4.17460431,5.1203717 L7.67460431,2.1203717 C7.86184918,1.9598761 8.13815082,1.9598761 8.32539569,2.1203717 L11.8253957,5.1203717 C12.0350586,5.30008277 12.0593394,5.61573277 11.8796283,5.82539569 C11.6999172,6.03505861 11.3842672,6.05933938 11.1746043,5.8796283 L8,3.15853889 L4.82539569,5.8796283 Z" id="Combined-Shape" fill="#000000" fill-rule="nonzero"></path>
+    </g>
+</svg>