(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/Pivot.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 / Pivot</title>
+    <desc>Created with Sketch.</desc>
+    <g id="Icons-/-UI-/-Pivot" stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
+        <path d="M3,1 L13,1 C14.1045695,1 15,1.8954305 15,3 L15,13 C15,14.1045695 14.1045695,15 13,15 L3,15 C1.8954305,15 1,14.1045695 1,13 L1,3 C1,1.8954305 1.8954305,1 3,1 Z M5.375,4.5 L5.375,4.78546341 L7.6346501,8.02270732 L5.375,11.0945122 L5.375,11.5 L10.625,11.5 L10.625,10.0328171 L9.82649901,10.7264146 L6.91973161,10.7264146 L6.91973161,10.4169634 L8.902833,7.61585366 L6.91973161,4.99085366 L9.77431213,4.98735366 L10.625,6.09531707 L10.625,4.5 L5.375,4.5 Z" id="Combined-Shape" fill="#000000" fill-rule="nonzero"></path>
+    </g>
+</svg>