(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/FieldFunctionEqual.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 / FieldFunctionEsqual</title>
+    <desc>Created with Sketch.</desc>
+    <g id="Icons-/-Fields-/-FieldFunctionEsqual" stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
+        <path d="M5.5,6 L10.5,6 C10.7761424,6 11,6.22385763 11,6.5 C11,6.77614237 10.7761424,7 10.5,7 L5.5,7 C5.22385763,7 5,6.77614237 5,6.5 C5,6.22385763 5.22385763,6 5.5,6 Z M5.5,9 L10.5,9 C10.7761424,9 11,9.22385763 11,9.5 C11,9.77614237 10.7761424,10 10.5,10 L5.5,10 C5.22385763,10 5,9.77614237 5,9.5 C5,9.22385763 5.22385763,9 5.5,9 Z M4,3 C3.44771525,3 3,3.44771525 3,4 L3,12 C3,12.5522847 3.44771525,13 4,13 L12,13 C12.5522847,13 13,12.5522847 13,12 L13,4 C13,3.44771525 12.5522847,3 12,3 L4,3 Z M4,2 L12,2 C13.1045695,2 14,2.8954305 14,4 L14,12 C14,13.1045695 13.1045695,14 12,14 L4,14 C2.8954305,14 2,13.1045695 2,12 L2,4 C2,2.8954305 2.8954305,2 4,2 Z" id="Combined-Shape" fill="#000000" fill-rule="nonzero"></path>
+    </g>
+</svg>