(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/FieldDate.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 / FieldDate</title>
+    <desc>Created with Sketch.</desc>
+    <g id="Icons-/-Fields-/-FieldDate" stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
+        <path d="M5,4 L11,4 L11,2.5 C11,2.22385763 11.2238576,2 11.5,2 C11.7761424,2 12,2.22385763 12,2.5 L12,4 L12.5,4 C13.3284271,4 14,4.67157288 14,5.5 L14,12.5 C14,13.3284271 13.3284271,14 12.5,14 L3.5,14 C2.67157288,14 2,13.3284271 2,12.5 L2,5.5 C2,4.67157288 2.67157288,4 3.5,4 L4,4 L4,2.5 C4,2.22385763 4.22385763,2 4.5,2 C4.77614237,2 5,2.22385763 5,2.5 L5,4 Z M3,7 L13,7 L13,5.5 C13,5.22385763 12.7761424,5 12.5,5 L3.5,5 C3.22385763,5 3,5.22385763 3,5.5 L3,7 Z M13,8 L3,8 L3,12.5 C3,12.7761424 3.22385763,13 3.5,13 L12.5,13 C12.7761424,13 13,12.7761424 13,12.5 L13,8 Z" id="Combined-Shape" fill="#000000" fill-rule="nonzero"></path>
+    </g>
+</svg>