(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/FieldDateTime.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 / FieldDateTime</title>
+    <desc>Created with Sketch.</desc>
+    <g id="Icons-/-Fields-/-FieldDateTime" stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
+        <path d="M8.5,7.5 L11.5,7.5 C11.7761424,7.5 12,7.72385763 12,8 C12,8.27614237 11.7761424,8.5 11.5,8.5 L8,8.5 C7.72385763,8.5 7.5,8.27614237 7.5,8 L7.5,4.5 C7.5,4.22385763 7.72385763,4 8,4 C8.27614237,4 8.5,4.22385763 8.5,4.5 L8.5,7.5 Z M8,14 C4.6862915,14 2,11.3137085 2,8 C2,4.6862915 4.6862915,2 8,2 C11.3137085,2 14,4.6862915 14,8 C14,11.3137085 11.3137085,14 8,14 Z M8,13 C10.7614237,13 13,10.7614237 13,8 C13,5.23857625 10.7614237,3 8,3 C5.23857625,3 3,5.23857625 3,8 C3,10.7614237 5.23857625,13 8,13 Z" id="Combined-Shape" fill="#000000" fill-rule="nonzero"></path>
+    </g>
+</svg>