(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/UI/DragDrop.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 / UI / DragDrop</title>
+    <desc>Created with Sketch.</desc>
+    <g id="Icons-/-UI-/-DragDrop" stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
+        <path d="M6.5,3 C6.77614237,3 7,3.22385763 7,3.5 L7,12.5 C7,12.7761424 6.77614237,13 6.5,13 C6.22385763,13 6,12.7761424 6,12.5 L6,3.5 C6,3.22385763 6.22385763,3 6.5,3 Z M9.5,3 C9.77614237,3 10,3.22385763 10,3.5 L10,12.5 C10,12.7761424 9.77614237,13 9.5,13 C9.22385763,13 9,12.7761424 9,12.5 L9,3.5 C9,3.22385763 9.22385763,3 9.5,3 Z" id="Combined-Shape" fill="#000000" fill-rule="nonzero"></path>
+    </g>
+</svg>